Wellington Water is working at close to full capacity with two major pipelines in the city failing within a month of one another.

Mayor Andy Foster and Wellington Water chief executive Colin Crampton on Friday fronted media over the city's poo-mageddon debacle.

Just before Christmas a wastewater pipe collapsed under Willis and Dixon Streets, necessitating the diversion of up to 100 litres of waste a second into the harbour.

It's estimated five million litres of wastewater flowed into the harbour before an above-ground bypass pipe could be put in place. That's the equivalent of two Olympic-sized swimming pools.

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Another pipeline failed just last week in a wastewater tunnel beneath Mt Albert.

That has resulted in more than a million litres of sludge being transported every day by truck from Moa Point Treatment Plant and the landfill at Carey's Gully until the pipeline can be repaired, which could take a month or even longer.

Wellington Water chief executive Colin Crampton said they were close to full capacity dealing with the two incidents.

"We would be struggling if anything else was to happen."

At this stage Wellington Water hasn't needed to bring in any extra help for the jobs with local experts at hand.

Wellington mayor Andy Foster thanked those who've worked around the clock on the Willis St site. Photo / Georgina Campbell.
Wellington mayor Andy Foster thanked those who've worked around the clock on the Willis St site. Photo / Georgina Campbell.

Wastewater work is specialised as it's complex, risky, and the infrastructure is deep underground.

Willis St is expected to reopen in April after a permanent bypass pipe is trenched in.

It's still not known what went wrong in the wastewater tunnel beneath Mt Albert.

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The sludge pipeline from the treatment plant to the landfill runs for 9km. It consists of two pipes, which operate one at a time, to allow for maintenance.

Wellington Water considers it highly unusual that both have failed.

Instruments will be sent into the tunnel or the pipe itself to locate the defect, which is estimated to be 200m inside.

"Once we've located that we can work out whether we can fix it by using the pipes themselves, sort of like putting a stent in you arteries in your body, or do we have to empty the tunnel and get in there and do a bigger fix", Crampton said.

There was a risk that if the trucking operation was interrupted, some sludge may need to be discharged via a long outfall pipe that carries treated wastewater out to Cook Strait.

But Crampton said the situation had moved from "delicate to robust" and he was confident sludge would not end up in the ocean.

Wastewater is flowing through an above ground bypass on Willis street after a pipe collapsed just before Christmas. Photo / Georgina Campbell.
Wastewater is flowing through an above ground bypass on Willis street after a pipe collapsed just before Christmas. Photo / Georgina Campbell.

"We do not want any of that stuff in the sea that's our fundamental objective all the time", Crampton said.

The trucking process has been streamlined and now only six vehicles are needed for the operation instead of twelve.

Mayor Andy Foster said it was "appallingly bad luck" the incidents had happened in such close proximity to one another.

He said all the stops were being pulled out to get the infrastructure failures fixed quickly.

It was possible there were still underlying issues from the 2016 Kaikoura Earthquake, he said.

"It took a period of time before we uncovered some of those things in respect of the buildings above ground, it's possibly not surprising that there might be some things underground that we maybe didn't pick up immediately and that have come to light over a period of time."

Foster was comfortable with the council's current spend on water infrastructure which amounted to about $180m a year.

It's a figure that is expected to increase as pipes age but Foster stressed it was a balancing act and he didn't want to replace anything prematurely.

Crampton said the council set investment levels and it was Wellington Water's job to work inside those budgets.