The Western Hills main sewer runs from Silverstream, through Upper Hutt, along State Highway 2 near the Hutt River through to Petone and then to the Seaview Wastewater Treatment Plant, pictured. Photo: Google Maps
A sewerage pipe that could collapse State Highway 2 has been given the highest possible risk rating by Wellington Water, documents show.
A corroding sewerage pipe could collapse a section of State Highway 2 in Lower Hutt if it fails, new documents reveal.
In April, RNZ reported the 60-year-old pipe - which carries all of Upper Hutt's sewage - could be at risk in two places.
Authorities are now trying to work out how to fix a section under the major throughfare.
Documents released under the Official Information Act reveal the scale of the damage the pipe could cause and the estimated $4.5 million fix.
The Western Hills main sewer runs from Silverstream, through Upper Hutt, along State Highway 2 near the Hutt River through to Petone and then to the Seaview Wastewater Treatment Plant.
The pipe is at risk in two places near Taita Rock and under State Highway 2 near Melling Station.
An urgent briefing on the pipe said its risk rating near Melling Station was 25 out of 25 due to sections of it being in poor condition.
There was also a risk that "significant structural failures will increase".
The document said that there was a high risk of failure and collapse due to ongoing corrosion from hydrogen sulphide gas.
If the pipe failed, it could cause large volumes of sewage to flow into the Hutt River for "extended periods "and trigger the collapse of a section of State Highway 2 that sits above the pipeline.
The document also stated a failure could result in significant disruption on the road because of emergency works and disrupt wastewater services to both Lower and Upper Hutt.
It also noted that if the pipe continued to degrade, it may have to be replaced rather than just re-lined, which Wellington Water estimated would cost about three times as much.
Wellington Water told RNZ it had created a contingency plan for if the pipe did fail.
The fix
The cost of relining the pipe, its funding risks and other associated works such as traffic management for State Highway 2 would total $4,572,523, the documents showed.
Future design concept works should consider renewing the pipe versus relining it, the briefing said.
The work for the pipe was unfunded and would need to be added to Wellington Water's wastewater renewals budget.
In response to questions, Wellington Water told RNZ the $4.5 million price tag was subject to change depending on the options assessment and an ongoing design process.
Hutt South MP Chris Bishop told RNZ that he expected Wellington Water, the New Zealand Transport Agency Waka Kotahi, and the relevant local councils to be treating the problem "seriously".
Lower Hutt Mayor Campbell Barry said the information provided to RNZ would come through to the council in due course.
"There will be options as we consider it alongside our capital investment programme, which we signed off in our long term plan last month."
Greater Wellington Regional Council chair Daran Ponter said the group's concern was ensuring that no sewage discharge entered Te Awa Kairangi or its tributaries if a failure was to occur.
"This could become problematic in a high rainfall event."
He said NZTA may need to be prepared to limit traffic under State Highway 2 while remedial works took place.