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Westport water trunk main replacement

Kia ora koutou - welcome to the project page about the Westport water trunk main replacement. This upgrade is part of Council's infrastructure work programme to improve the resilience of local water supplies.


The Westport water supply trunk main replacement project is a key infrastructure project to replace the aging trunk main water pipes supplying Westport and Carters Beach residents with drinking water.

The existing gravity-fed spiral steel trunk main was built in 1961. It has exceeded its serviceable life and therefore poses a risk in terms of reliability, resilience, and compliance. The replacement project is being completed in three stages – the terrace section pipeline (downhill from the reservoir), the flat section in Westport to McKenna Road and the section from McKenna Road to Queen Street.

The trunk main has been replaced from the reservoir to McKenna Road in Westport. The old 14” steel pipe which has approached the end of its effective service life has been replaced with new, resilient polyethylene (PE) pipe.

As a flexible pipeline, PE can withstand a greater seismic event and as a key piece of infrastructure is expected to remain intact during a major event. The pipeline also has a greater capacity, allowing for future growth in Westport.

Currently the pipeline has been commissioned from the reservoir to McKenna Road. Since the new pipe has been installed at the terrace section, Westport has seen an approximate 20% reduction in water usage.

Along with the pipeline installation a Pressure Reduction Valve (PRV) has also been installed. A key part of the overall strategy to reducing Westport’s leakage is to lower the water pressure within the town’s pipe network. Lowering the pressure, will both reduce leakage rates through existing leaks but also lower the burst frequency of pipes within the reticulation.

There are still parts that need to be completed to replace to entire trunk main. This will be subject to the Long-Term Plan 2024 – 2034 process that will seek to endorse further funding.

Once the entire length of the trunk main is replaced it will ensure a fully resilient water supply for Westport and Carters Beach.

The Pressure Reducing Valve has been installed and Council is planning the operational phase. The reduction of the township pressure is due to commence in mid-2023. Data is currently being obtained from the reticulation network to plan the pressure reduction.

The section of pipe from the south side of Scott’s Bridge to McKenna Road was completed in September 2023. The final stage of the project from McKenna Road through to Queen Street started at the end of July 2023. Upon completion of this work, approximately 4.4 km of pipeline from the reservoir into the township will have been replaced.

700m of pipe along the terrace section and 2km of pipe through to Scotts Bridge has been funded through $4M of Department of Internal Affairs (DIA) funding. These sections were installed by WestReef Services Ltd and Hadlee & Brunton Ltd.

The $4M DIA funded work also incorporated installing the pressure reduction valve equipment.

The replacement of the 1.5km pipe section from Scotts Bridge to Kew Rd is part of Council’s $1.6M investment into Westport’s water infrastructure included in the 2022-2023 Annual Plan.

For any queries regarding this project, please send an email to

Picture: Map of Westport Water trunk main with completed sections.

Westport water upgrade

Read about the background on the Westport water upgrade.

The original Westport Water Supply was opened in 1903 after a lengthy construction period, when four tunnels and timber water races were established between the south branch of Giles Creek (headwaters of the Orowaiti River) and the site of the raw water reservoirs.

Our forefathers had the great insight to build a gravity feed water supply for the township. They created tunnels at such a gradient that allowed the water to flow naturally but without scouring when the volume of water increased during rain.

This water was piped to the Westport township by a single eight inch (200mm) diameter cast iron pipeline. Around the mid 1920s a second 14 inch (355mm) diameter steel delivery pipeline was installed to provide additional water to cope with increased population.

This supply system continues to supply Westport with water to the current day.

In 1967 a delivery pipeline was installed by Carters Beach residents between Westport and Carters Beach, allowing Carters Beach to join the water supply.

In 1986 Westport Borough Council constructed a water treatment plant which at that time was state of the art and graded Aa (the highest grading possible). At this time there was controversy as the whether Council should make this upgrade or not, there were doctors with opposing comments about the need to carry out the upgrade etc but in the end the Council of the time made the decision to carry out the upgrade. A subsidy from the Government was available at this time so Council was able to utilise those funds towards the project.

During the 1986 construction of the Water Treatment Plant Council invested $1 million in this project, which relayed into today’s terms is approximately $2.5 million.

In the year 2000 the drinking water standards of New Zealand were changed. Since these changes came into force the Westport Water Supply has remained un-graded, as the current system does not meet that standard.

Also in 2000 Council experienced its first major tunnel collapse of the tunnels that were constructed in the early 1900s. An outcome of this collapse was tat Connell Wagner were commissioned to analyse all possible options for supplying water to Westport. 

Outcomes of this work included a range of options including:

  • A pumped supply from Gillows Dam
  • A pumped supply from Reedy’s Road bore field
  • A pumped supply from Lower Orowaiti River
  • A pumped supply from Upper Orowaiti River
  • Maintain existing supply & pipe existing tunnels
  • Maintain existing supply & refurbish existing tunnels

A supplementary report provided information on the following:

  • Maintain existing supply & construct new tunnels.

The most cost effective option was to keep the existing supply but to add an alternative supply, this working as a backup to the main system removing the risk of depending on the tunnels. Also giving a back up option during a time of drought. The addition of the alternative water supply cost the Council approximately $1 million.

A major collapse in the number four tunnel in 2009 resulted in the tunnel being piped the whole length. This 400 metre long tunnel had caused issues on several occasions over many years. Therefore it was considered that piping the tunnel was the best option, this cost approximately $650,000. While work was being carried out in the tunnel, the alternative water supply proved invaluable as the water was pumped from the alternative supply during the entire time that this work was being carried out which took approximately eight months.

Council has spent several years investigating affordable options for the Westport Water Supply to meet the current Drinking Water Standard, which after the initial changes in 2000, have seen two further rounds of amendment or change in 2005 and 2008.

During the 2013/2014 year the Water Treatment Plant was upgraded to meet the current New Zealand Drinking Water Standards.

The current water treatment processes at the Westport Water Treatment Plant (WTP) comply with the current Drinking Water Standards for Chemical composition, and the treatment of bacteria, however it does not comply for the treatment of Protozoa.

Protozoa is a waterborne bug that is so small it cannot be seen by the naked eye, and it is difficult to remove by filtering. It can remain dormant in the water supply, but if ingested through drinking water, will quickly multiply and thrive in the warm conditions of the gut, causing moderate to severe sickness which can last for several months.

The introduction of Ultra Violet (U.V.) treatment will sterilise Protozoa bugs, so they cannot reproduce and will pass harmlessly through the gut.

The turbidity metre will detect changes in the raw water turbidity and the Water Treatment Plant will automatically alter the chemical dosing regime to adequately treat the changing water conditions.

The construction of the Flocculation tank will allow longer retention time for the formation of larger Floc particles. These are more easily filtered out.

The backwash tank will retain sufficient water for regular programmed backwashing of the water filters.

Installation of the U.V. facility allows the sterilisation of protozoa which as previously mentioned was untreated and harmful if ingested through drinking water.

Increased capacity to store treated water will provide greater Plant efficiency and limit the effect of plant outages, such as equipment malfunctions, and power failure by having more treated water available.

What all these terms mean:

  • Turbidity – Water particles (low=clear water and high= dirt/muddy water)
  • Coagulant – Chemical additive to combine water particles together, this makes filtration easier
  • Flocculation tanks – Where particles combine together to form larger particles
  • Filters – Removes Flocculation particles
  • U.V. (ultra violet light) – Sterilises bugs and other nasties that are in the water
  • Chlorine – Kills all bugs in water except for Protozoa
  • PH correction – To remove acidity in water (reduce pipe corrosion)

What has been considered?

Opus International Ltd consultants were engaged to complete a peer review of the strategic study that was carried out at an earlier stage by Connell Wagner, conduct an operational review of the current Westport plant, and assess any alternative options Council could consider.

Throughout all of the research conducted two areas were considered worth pursuing, to continue with the existing system or to go to the Buller River.

From this report there were three options:

Option 1 – Upgrade the current supply costing $11 million

Option 2 – Go to the Buller River with a Gravity system costing $15 million

Option 3 – Go to the Buller River with a pump system costing $13 million

The gravity system in the Buller River proposed to pump water from an infiltration gallery next to the river to an elevated reservoir above Te Kuha which would then be gravity fed to the reticulation.

The proposed pump system in the Buller River would pump water from the Nine Mile area above the tidal mark, this water would be pumped to a new Water Treatment Plant which would be located around the Holcim packing plant on Nine Mile Road then be pumped to the reticulation.

Council decided to upgrade the current supply as the capital expenditure was identified along with the average running cost over a 25 year period.

In 2012 Council completed it’s Long Term Plan which details plans for the next ten year period, to be reviewed in three years time. In this document there is a detailed plan of when work would be carried out on the Westport Water Supply upgrade.

After the 2012 Long Term Plan was approved, Buller District Council received a subsidy from the Ministry of Health of $1,474,250 to go towards the upgrade, making the Westport Water Supply meet the NZ Drinking Water Standards.

Council has had to re-plan when this work will be carried out as the subsidy is only available during a certain time period, so all items that are being subsidised need to be completed while the funding is available. the revised programme is now as follows:


Planning and design


Raw water quality management

Flocculation tanks and foundation

Filters refurbishment and seismic upgrade

Provision of treated water storage for backwash filters

Filter to waste system

Upgrade buildings, monitoring and control

U.V. disinfection and control

Alkalinity correction

Bypass raw water ponds

New treated water reservoirs

Total cost for year:$5,604,200


Replace trunk main

Raw water ponds clean out

Total cost for year: $4,237,000

To be considered at a later date: Lining of the raw water ponds and piping tunnels 1, 2 & 3.