Work is being accelerated to fix a critical point in a massive wastewater tunnel running underneath Wellington after severe corrosion was discovered, jeopardising its structural integrity.
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.
Average waste flows at the location are estimated to be about 800 litres per second.
It's just one of several woes which have recently "stretched" Wellington Water to full capacity.
The section 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.
Since then it has been extended several times to accommodate Wellington's growing population and it now stretches 23km from Ngauranga to the Moa Point Treatment Plant.
Wellington Water carried out an inspection late last year and found the section near the plant was in worse condition than first thought.
The area of concern is largely under Moa Point Rd and Stewart Duff Drive and has been subject to severe corrosion jeopardising its structural integrity.
Wellington Water's Mark Kinvig said a close eye has been kept on the section over the past two years, including condition assessments.
The top of the pipe was being subjected to a hydrogen sulphide "attack", he said.
"The gas that comes off from the sewage becomes sulphuric acid and then eats through the concrete, and eventually into the soil, if left unattended."
Modern technology meant a robot could install a liner inside the pipe and this refurbishment would be completed by the end of the current financial year, Kinvig said.
In its consent application for the work, Wellington Water said it was understood continued corrosion could cause the pipe to fail spontaneously.
This would cause a sewage spill that could potentially generate significant disruption, reputational, environmental, and cost impacts due to the asset's critical nature.
"Asset failure would see significant uncontrolled localised discharge at the southern end of the airport, and further uncontrolled discharge toward Lyall Bay and Kilbirnie."
How Wellington's wastewater network was built
Wellington's population rapidly grew in the late 19th century after becoming the capital city in 1865.
There were increasing cases of typhoid and cholera which were attributed to poor sanitation.
After some political deliberation, construction on a waste-water network began in 1893. It cost £175,000 and was completed by 1899.
A large pipe to take wastewater from Manners St through Mt Victoria and out to the south coast was known as "the interceptor".
In the 1930s the interceptor was extended from Manners St through to Pipitea St, as the city's population topped 100,000.
A separate wastewater system was built in Karori.
As suburbs spread further north to the likes of Johnsonville, another extension of the interceptor was made from Ngauranga Gorge and through Ngaio Gorge to connect at Thorndon.
Wellington Water has previously reported that population projections have exceeded the original interceptor design, but the way it was constructed meant the interceptor has been large enough to accommodate the expanding city.
That's because most pipe shapes used over the years were at least six feet high and three feet wide, which allowed miners to stand inside the tunnel and wield their picks.
The interceptor used to be inspected manually by workers who walked through it when flows were low, but this is now done with high-definition video cameras.
Some of the infrastructure is now more than 120 years old, but every day 67 million litres of waste water flows through the interceptor to Moa Point for treatment.