A large sinkhole that opened up in Auckland's St Mary's Bay will be complex to fix, Watercare says, because the damaged wastewater pipe is five metres underground - below large transmission pipes.

The hole emerged a week ago and is about five metres long by three metres wide and over a metre deep in parts.

It is in a part of Hackett Street that was badly damaged by flooding three years ago and had been covered by a steel plate since then.

Watercare's chief operations officer Mark Bourne said fixing it could take up to two months and about 10 metres of wastewater pipe needs to be replaced.

"Hackett Street has a large amount of underground infrastructure, which makes work like this challenging," he said.

"The street has both large transmission pipes and smaller local water and wastewater pipes at varying depths underground, which means we have to plan repair work carefully to make sure we don't affect other infrastructure."

Bourne said the damaged wastewater pipe is five metres underground and located beneath large transmission pipes, which will make the repair complex and technically challenging.

"We are in the process of engaging a specialist contractor to start work on the damaged wastewater pipe. This is expected to be a significant project because of the depth of the pipe and the complexity of the site."

When the sinkhole appeared, residents from the St Mary's Bay Association raised concerns about the area's ageing water infrastructure.

It comes as Watercare plans to spend $15.2 billion on Auckland's water and wastewater infrastructure over the next 10 years, resulting in an average household's water bill rise of about $8 a month.

Tony Skelton has lived in St Mary's Bay for 53 years and said the repair work started at midnight on Tuesday and has continued since.

"We have a reasonable amount of confidence that this time they're going to address the underlying issues with the emergency overflow point and give us some peace of mind that it will be fully and totally repaired."

He said there is traffic and noise disruption and a large repair site that is cordoned off.

"We're not complaining, it's get the job done as far as we're concerned. We as residents are comfortable with some disruption on the basis that we're going to get the problem resolved."

The wastewater infrastructure in the area dates from the 1910s.

Bourne said it is necessary to do the pipe repairs.

"We really appreciate the community's patience while we do this complex work. We apologise for the inconvenience, and we will make every effort to get these repairs done and keep residents and key stakeholders updated on our progress," he said.

Hackett Street is home to drinking water, wastewater and stormwater infrastructure that serves both the local area and wider Auckland.

In the 2023 flooding, a small hole appeared in the road near 16 Hackett Street, later found to be caused by a failure in a deep wastewater pipe beneath other services.

"To ensure public safety, a steel plate was installed over the hole while investigations were undertaken. We discovered that there was damage to a local wastewater pipe," Bourne said.

"During this period, we did regular site inspections to monitor the area to make sure it remained stable and wasn't deteriorating."

Earlier this month, an unrelated leak occurred on a nearby drinking water transmission main and that has now been repaired.

Bourne said many clay pipes across the water network are in good condition despite their age.

"The pipes' age alone does not mean they need replacing. We decide when to renew them based on evidence of their condition and performance," he said.

"While there are some localised issues, the general wastewater network condition in St Mary's Bay is good."