A review has found those managing Wellington's water network effectively lost a 2004 report identifying corrosion in a wastewater tunnel that went on to collapse.

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

An independent review into the Dixon St wastewater tunnel incident has been quietly published on Wellington Water's website.

It reveals Wellington City Council commissioned a person-entry inspection of the interceptor and connecting adit in 2004.

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It identified an area of significant corrosion deterioration in the same place the tunnel later failed. The report recommended a further inspection on a 10-year cycle following repair work.

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A hard copy of the report was eventually archived in a project file, but was misplaced as responsibility for water management changed hands to a company called Capacity and then again in 2014 with the establishment of Wellington Water.

"Few staff in key wastewater network decision-making roles from the time of that study are still within Wellington Water and that information was effectively lost," the review said.

More recent condition assessments have been focused on the interceptor, a massive wastewater tunnel running underneath Wellington, into which the adit tunnel discharges.

The adit was not directly inspected due to changes in health and safety legislation, which have resulted in a move away from person-entry inspections making the adit difficult to reach.

Contractor staff discovered a large subsurface cavity above the adit in early December. Photo / Wellington Water.
Contractor staff discovered a large subsurface cavity above the adit in early December. Photo / Wellington Water.

But it was the adit that collapsed.

"Knowledge of the existence of the adit was limited amongst existing staff members, let alone an understanding of its condition", the review said.

The review listed management of information as one of the key things that led to the tunnel collapsing and which needed to be addressed to help prevent the same thing from happening again.

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It noted little was known about the asset despite this 2004 assessment, which identified it as being in poor condition.

It recommended the management of condition, failure and maintenance information relating to the network be improved so that information is "accessible and is not lost, even with staff turnover".

The independent review said the overall response to the incident was strong.

"It was a major and fast-moving incident which Wellington Water responded to with significant effort using an emergency-response approach."

Wellington Water Chief Executive Colin Crampton said the organisation accepted the findings of the review.

"We inherited a diverse mix of historical asset data from councils around the region, in a range of formats and systems, which we have been working to digitise and consolidate within a single platform, and that work is ongoing.

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"We have also implemented continuous condition assessment programmes to improve the overall quality of the data we hold.

"The review highlights that not all the condition information that existed at the time was transferred to Wellington Water on its establishment, which further underlines the importance of systematic asset data management."

Councillor Sean Rush says Wellington City Council has approved $400,000 in funding to accelerate Wellington Water's asset condition assessment work. Photo / Mark Mitchell.
Councillor Sean Rush says Wellington City Council has approved $400,000 in funding to accelerate Wellington Water's asset condition assessment work. Photo / Mark Mitchell.

The review also reported there was strong anecdotal evidence that increases in funding for asset renewal programmes were regularly sought from Wellington City Council, and regularly declined.

First term Wellington City Councillor Sean Rush, who holds the Three Waters portfolio for the city said in response: "Under my watch they've asked for money, and they've got it."

"Wellington City Council has approved $400,000 in funding to accelerate Wellington Water's asset condition assessment work, meaning the condition of all high-criticality assets in the city's drinking water and wastewater networks will be assessed within the current financial year."

Mayor Andy Foster, who has been on Wellington City Council for almost three decades, said he was not aware of any approach made to the council as an elected body asking for additional resource that was ever declined.

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However, he said he has been told there were approaches made to individual elected members.