A Watercare sewer project is out of pocket and months behind schedule after it hit natural gas lurking under a park in west Auckland.

The wastewater pipe project in Massey has been pushed back by six months after tunnelling subcontractors hit a deposit of methane and carbon monoxide 8 metres below the ground.

The Lawsons Creek Branch Sewer project in Massey involves duplicating a sewer system to increase capacity in booming West Harbour and Hobsonville.

Work began in February last year, with Abergeldie Complex Infrastructure boring a tunnel under Moires Park and neighbouring streets to lay the 450mm wastewater pipe.

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But in late October the workers' gas monitor unexpectedly picked up natural gas readings under Allington Rd.

Activity has halted while Watercare looks for an alternative route, meaning the project - meant to finish in June - won't be completed till December.

A Watercare spokeswoman confirmed to the Herald its subcontractor, Abergeldie Complex Infrastructure, was operating a small tunnel-boring machine underground when small pockets of methane and carbon monoxide were detected.

"The gas levels were minimal: methane gas levels: 0.25% [2500 ppm] and carbon monoxide: 0.04% [400ppm]," she said.

Work stopped and Worksafe was informed.

"The gases are believed to be natural occurrences, most likely caused by old rotting vegetation such as trees, deep underground."

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Before work began, Watercare had commissioned a geotechnical baseline report from environmental and engineering consultancy Tonkin & Taylor.

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It appears it did not pick up any gas readings.

The Herald understands subsequent testings has found gas across a wide area.

Dr Martin Brook, Associate Professor in Auckland University's School of Environment, told the Herald natural gas could be found in swamp deposits all over Auckland but finding it could be "hit and miss".

Ground investigations would normally pick up on hazards before excavation, but methane may not have been in the project's geotechnical risk register.

The fact work had stopped for three months suggested the deposit could be quite large, Brook said.

Auckland's geology was "unbelievably" varied.

"You've got volcanics, oscillating sea levels, peat deposits...that's why it's really important to get the ground investigation up front. But often things slip through the net," he said.

"It's not like doing an investigation in Sydney or Brisbane where you've got layer cake geology - it's so challenging here. That's why infrastructure development here costs so much."

The Lawsons Creek sewer duplication project has ground to a halt after workers discovered gas last October. Photo / Jason Oxenham
The Lawsons Creek sewer duplication project has ground to a halt after workers discovered gas last October. Photo / Jason Oxenham

The Herald also understands the contractors are still being paid despite work stopping.

Asked how much the delay was costing Watercare, the spokeswoman said the original project budget was $2.8 million, but all budgets had contingencies.

"Negotiations between Watercare and our contractors are confidential."

The cutter-head of the tunnel boring machine, owned by Abergeldie, would likely be abandoned and the tunnel back-filled with grout.

Asked whether the subcontractor would be reimbursed for its machine, the spokeswoman said payment had not been discussed as all options were still being investigated.

Watercare was looking for an alternative route, with the project due to finish in December.

WORKSAFE NOT INVESTIGATING

Watercare's spokeswoman said the workers had been using a small machine and did not need to be underground. They left the site when alarms sounded.

She said Watercare did not believe any workers were in any immediate danger; they were 30m from the gas source when the alarm sounded.

However the Herald has been told while the machine can be operated remotely, the workers were running it from underground.

WorkSafe told the Herald Abergeldie had reported the incident and work had stopped immediately, which was the appropriate response.

"Work has not resumed since the incident occurred. An investigation into the source of the gas is being undertaken by Abergeldie Complex Infrastructure and Watercare."

Worksafe was satisfied both businesses had managed the risk appropriately, and for that reason was not investigating.

The works have meant extensive road disruption on Allington Rd, where residents have been told of the delays but not the reason behind them.

The Watercare project has meant extensive road disruption for Allington Rd locals. Photo / Jason Oxenham
The Watercare project has meant extensive road disruption for Allington Rd locals. Photo / Jason Oxenham

Henderson ward councillor Linda Cooper had also not been alerted to the incident - but said she did not expect to be as it was unlikely to affect people's lives.

"It's an operational issue - they're a council controlled organisation and they've got a handle on it."

The sewer work was not urgent as it was being built to future proof capacity, and any rerouting was unlikely to affect housing or traffic, Cooper said.