Watercare is setting up an independent review into the cause of the massive sinkhole and sewer blockage at Parnell that has allowed about 8 million litres of wastewater to pour daily into Waitematā Harbour.

Watercare chief executive Dave Chambers today said the review, by an engineering consultancy, was expected to take several months and the full report would be made public.

“We’ve shared draft terms of reference for this review with the Waitematā Local Board, our board of directors, Ngāti Whātua Ōrākei and Auckland Council’s Healthy Waters department, and have taken into consideration the feedback provided,” Chambers said.

The collapsed sewer on St Georges Bay Rd in Auckland's Parnell is allowing untreated waste to pour into the Waitematā Harbour. Photo / Watercare

The review will:

• Look at all data from the sinkhole and develop a theory about the cause
• Examine Watercare’s historical and planned maintenance and condition assessment programmes
• Study the current capital programme for wastewater transmission sewer rehabilitation

Chambers said Watercare will confirm the name of the engineering consultancy when the agreement has been signed.

The sewer line collapsed and the massive sinkhole opened last month.

The 13-metre-deep hole descended to a 2.1m-wide collapsed brick pipe that served Central Auckland and West Auckland.

Watercare said bypass, due for completion in about five days, will divert wastewater around the blockage by pumping it between manholes, and will “end or significantly reduce” the sewage flowing into the harbour.

• Auckland sewer sinkhole: Millions of litres of wastewater to pour into harbour, three days to stem flow
• Auckland sinkhole: 13m-deep crater after sewer collapse, wastewater pours onto city beaches

A diagram showing the impact of the Parnell sinkhole on the Ōrākei main sewer. Photo / Watercare

An important milestone was reached on Tuesday with the installation of a “stoplog”, Watercare head of service delivery Sharon Danks said.

“The stoplog is basically a steel fabricated door that seals the sewer and prevents wastewater from entering the blockage site.

“It’s an essential component of the bypass project but it also means that, when we are in a position to finish clearing the blockage, we can do so safely – without a wall of wastewater on the other side.”