Up to 100 litres of contaminated water per second gushed into Wellington Harbour after a wastewater tunnel collapsed in Willis St.

Wellington Water is sitting on a backlog of nearly 1000 leaks in pipes dating back to the Great Depression as it scrambles to deal with a massive wastewater leak into the harbour.

The state of Wellington's pipe work has been highlighted after a wastewater leak in 1930s pipes on Friday shut down a chunk of the central city and saw a swimming pool worth of dirty water, including faeces, washed into Wellington Harbour daily.

The leak, caused by an underground tunnel collapsing, was reduced from 100 to 10 litres per second by Sunday morning after a disused 1890s pipe was recommissioned.

ROSS GIBLIN/STUFF Part of Willis St was closed for the urgent works.

But, Wellington Water has confirmed that it is currently sitting on a backlog of 920 leaks in its water system across Wellington, Hutt Valley, and Porirua. It has an average repair time of 21 days, which is outside of council targets, and the number of reported leaks had increased from about 10,000 in 2014 to almost 16,000 in 2019.

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Wellington City councillor Sean Rush, who recently inherited the three waters portfolio, confirmed the 80-year-old tunnel that collapsed was symptomatic of the wider network across tap water, wastewater, and stormwater – collectively known as the "three waters".

ROSA WOODS/STUFF Wellington City councillor said Wellington's pipework was nearing the end of its life.

Their age, combined with damage sustained during the Kaikōura quake, meant there had been a "significant increase" in losses.

At least 20 per cent of Wellington's tap water was lost to leaks, but a lack of water metering meant it was hard to get an accurate picture of losses, he said.

Wellington's pipes were coming to the end of their lives and "how we deal with it is still being considered", he said.

ROBERT KITCHIN/STUFF The water spouting from Wellington's Oriental Bay fountain was picturesque but poisonous.

Replacing the entire network in one go was not feasible because of the massive disruption and cost ,so he suspected the council would land on a 20 to 30-year programme of replacement.

"It seems to me we need to rank the needs on a suburb by suburb [basis]."

He was unsure how the work would be paid for, though believed a public-private partnership should be considered.

ROSS GIBLIN/STUFF Workers toiled through the night to reduce the fallout.

Council may need to reprioritise its spending but that didn't necessarily mean taking money away from nice-to-have projects such as a movie museum or convention centre, he said.

Wellington Water chief executive Colin Crampton said there were many pipes beneath Wellington made of asbestos and cement, cast iron, and earthenware. The number of complaints about leaks had increased since the 7.8-magnitude quake in Kaikōura, suggesting that was partly to blame.

"All of them are coming to the end of their useful life."

ROSS GIBLIN/STUFF Workers have reduced the amount of wastewater going into the harbour from 100 litres per second to 10 litres per second after a waste water tunnel collapsed in Willis St.

The Dixon-Willis St tunnel collapse on Fridat happened at a critical juncture where multiple pipes connected with one central conduit that took wastewater from Ngāūranga to the Moa Point treatment plant near Wellington Airport.

There was a similar-era piece of critical pipework in Thorndon near Sar St that Wellington Water would be looked at soon.

Wellington Water fixed all the leaks it was able to in the past financial year but was forced to carry 400-odd into the current year, he said.

Work to fix major leak continues

Wellington's wastewater leak has shrunk to one-tenth of its size after workers toiled through two nights to fix it.

The block on water activities placed over the harbour on Friday should be lifted within a week.

The leak, previously spilling 100 litres of wastewater per second into the harbour, was reduced to 10 litres per second today by diverting wastewater through a disused 1890 pipe beneath Willis St, Wellington Water chief executive Colin Crampton said.

An overground pipe was being installed to take the strain off the 1980 pipe, and would remain down Willis St for some time.

The section of Willis St between Dixon and Ghuznee streets would remain closed until work was completed.