The combined bill to fix Wellington's water woes amounts to as much as $25m - a figure which is expected to increase further still.

The most costly repair job is the work to reinstate two sludge pipes beneath Mt Albert, which both failed at the beginning of this year.

Wellington Water has estimated the repair cost could be as much as $5m and the sludge trucking cost up to $11m, assuming repairs are successful by mid to late May.

Wellington City Council owns the infrastructure and gives money to Wellington Water to maintain it and manage water services.

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The work to trench a new wastewater pipe underneath Willis St after a tunnel collapsed just before Christmas is forecast to cost up to $4m.

However, a connection to the main wastewater trunk line is yet to take place and the cost of the Covid-19 lockdown is yet to be accounted for.

Meanwhile, a preferred option for decommissioning the tunnel that actually collapsed is yet to be decided, so it's too early for Wellington Water to provide a cost range for that work.

A sample of sludge. Photo / Georgina Campbell.
A sample of sludge. Photo / Georgina Campbell.

Across to the south coast, work to reline a critical section of a massive wastewater pipeline at Moa Point will cost about $5m.

But there is still work to be carried out and Wellington Water is yet to fully account for the costs of work done during the Covid-19 lockdown, details of which will be provided to Wellington City Council as they come to hand.

Willis St's Olympic-sized wastewater leak

'Twas practically the night before Christmas when Wellington's water woes began.

A tunnel collapsed under Willis and Dixon Sts on December 20 last year and five million litres of wastewater was diverted into the harbour before an above-ground bypass pipe could be put in place.

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Willis St fully reopened rather uneventfully in the thick of Covid-19, after the successful connection of a new underground wastewater pipe replacing the temporary bypass.

But there is still work to do.

Crews have started on new storm and wastewater pipes and connections for Dixon St, which is expected to take up to two months.

After that, Wellington Water expects to open the part of the street between Willis St and MacDonald Terrace to at least one lane of traffic.

Wastewater flowed through an above-ground bypass on Willis St for months after a wastewater tunnel collapsed just before Christmas. Photo / Georgina Campbell.
Wastewater flowed through an above-ground bypass on Willis St for months after a wastewater tunnel collapsed just before Christmas. Photo / Georgina Campbell.

The wastewater tunnel that collapsed also still has to be decommissioned and a final option will be agreed on over the next couple of weeks.

This will allow for detailed design work, a timeline and costs to be confirmed.

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The interceptor's 120-year history

Work is underway to install a new and more resilient lining at a critical point in a massive wastewater tunnel running underneath Wellington after severe corrosion was discovered, jeopardising its structural integrity.

This project has largely flown under the radar during poo-gate because the area of concern hasn't actually failed yet.

The repercussions of such a failure would be quite frankly horrifying, that's because average waste flows at the location are estimated to be about 800 litres per second.

Failure to rehabilitate the 250 metre section of concern as soon as possible would risk it spontaneously failing, resulting in "catastrophic" health and environmental consequences, a resource consent application for the work said.

The section under Moa Point Rd and Stewart Duff Dr is part of a much larger tunnel, known as "the interceptor", which was first built in the late nineteenth century in response to increasing cases of typhoid and cholera.

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It provides a fascinating insight into how Wellington's water network was built.

To fix the corroded section, wastewater has been pumped and diverted to allow for the relining machine to operate within the pipe.

Work will continue over the coming weeks to stabilise the space between the lining and the damaged pipe.

The issue was discovered late last year during an inspection when Wellington Water discovered the pipe was in worse condition than first thought.

Sludge, sludge, sludge

A team of experts were called in from Germany in the middle of a global pandemic to fix two high-pressure sludge pipes, which failed beneath Mt Albert in January.

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It triggered a 24/7 trucking operation to transport sludge to the landfill to prevent it from going into the ocean.

About 100 trips are made every day to do this at a whopping cost of $650,000 a week.

Trucks are doing the work of a pipe that broke in January by transferring sludge between Moa Point treatment plant and the landfill at Carey's Gully. Photo / Mark Mitchell.
Trucks are doing the work of a pipe that broke in January by transferring sludge between Moa Point treatment plant and the landfill at Carey's Gully. Photo / Mark Mitchell.

It's the combination of a hefty price tag and stinky operation which has pushed this project into the limelight.

The suggestion sludge could be discharged into the Cook Strait instead of the trucking operation only added fuel to that fire.

But over the weekend, Wellington Water installed a high-strength liner through the first of two 1.8km sludge pipelines.

The operation took more than 9 hours, led by the Germany-based team that manufactured the liner and who were given special permission to enter New Zealand amid Covid-19 restrictions.

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A four-tonne capacity winch was set up at one end of the pipeline to pull through the liner.

The Moa Point Treatment Plant in Wellington. Photo / Mark Mitchell.
The Moa Point Treatment Plant in Wellington. Photo / Mark Mitchell.

Over the next two weeks the pipe will be reconnected to the network and comprehensively tested.

If there are no problems, the sludge trucking operation can finally cease, bringing relief to residents who've had to put up with the stench, and to the city council which has had to shell out for the operation.

The liner for the second pipeline has been made and over the past month has been described several times as making its way to New Zealand "soon".

The technicians, who spent two weeks in quarantine in Auckland before coming to Wellington, will remain in the country until that liner arrives and is installed.

It's worth sparing a thought for these international experts who have left their families in the middle of a pandemic, flown across the world and put their own health at risk, all in the name of Wellington's water woes.

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