It could take a week-and-a-half to fix a blocked sewer main at the bottom of a huge sinkhole in Auckland, Watercare warns.

Large volumes of raw sewage have been pumping directly into the Waitematā Harbour since the sewer line in Parnell collapsed, creating the 20m-wide and 13m-deep sinkhole on Monday.

On Thursday morning, Ngāti Whātua Ōrākei placed a rāhui across the Waitematā Harbour due to the deluge of sewage and its environmental impacts.

Watercare chief of operations Mark Bourne said crews worked overnight trying to unblock it, but it turned out to be much more clogged than initially realised.

"Unfortunately, it's not good news," he told Midday Report. " The sewer is completely blocked for around 25m, so this will take some time to unblock."

Some progress was made overnight using hydro excavation - essentially, high-pressure water blasting - but he said it would probably take 10 days to completely unblock. In the meantime, Watercare was implementing a bypass.

"What we're planning on doing moving forward is continuing with the hydro excavation work, but we recognise that that may well take quite some time if that was our only means of unblocking it.

"So the other thing that we're determined to progress with is introducing what we call bypass pumping. So what that means is we will draw wastewater from upstream of where the blockage is, put in a new pump and lay a new series of pipes to be able to transmit wastewater around the blockage."

"And the purpose of that - much like unblocking it - means we can eliminate overflows."

He said it was the single worst blockage he had seen in his 35-year career. He said Watercare was not aware of any other sinkholes or blockages, but will "act… immediately" if one emerges, which is entirely possible considering Auckland's weather of late.

"The nature of this failure is very similar to other failures that we've seen, which were a direct result of flooding and storm events that have happened throughout this year."

Parnell's sinkhole is the city's third in recent months.

Dozens of beaches in the inner city have been closed for swimming, due to the risk of contamination from the sewage.

Sinking sales

Construction around the sinkhole has blocked the entrance to a few small businesses, forcing customers to climb make-shift stairs over a wall to reach their doors.

One of those businesses affected was Top Notch Bodyworks. Clinical director Anj Young said the wall was a barrier for customers, especially when they were injured and hoping to see a specialist like her, an osteopath.

"We have had to cancel quite a lot of our patients because of the fact that they're not able to climb those stairs and, and get access to the premises."

Young said they were forced to cancel at least 10 to 15 patients, which had put a financial dent into the business.

"The advice we've been told by Watercare is to contact our insurance for reimbursement for lost business," she said. "Some of us don't have that type of insurance, so we will have to lump what that situation is."

On Wednesday, construction workers were in the process of building a ramp made of scaffolding materials to reach the businesses. Young hoped it would mean easier access for patients from now on.

Neighbouring business Social Cooking was in a similar situation, where construction had blocked the driveway, making it tougher for essential deliveries to get to the door.

Owner Katrina Horton said it had made it tougher for essential food deliveries to get to her door. She said it was lucky the sinkhole was in a carpark.

"If it had been just a matter of metres in the other direction, it could have been a situation far worse than what it currently is."

With 85 people planned to arrive at the business at lunchtime on Thursday, Horton hoped there would not be any more spanners thrown in the works.

"We've got food deliveries and so on that need to happen," Horton said. "The big concern is, if it gets worse, whether it's actually safe for people to come into the premises at all."

Horton said she was not impressed with the strength of the sewage system.

"It is horrifying that things like this can happen in the middle of New Zealand's biggest city. No one expected this to happen. It was a little surprising to find out the age of the sewage system and to hear how big the sinkhole was."

Some of the pipes in the area date back to the 1920s.

Both Horton and Young said Watercare had been fantastic at communicating with the people in the area.

Craig Horne from nearby cafe Axis Espresso said the sinkhole had become the talk of the neighbourhood.

"There's a bit of worry, a few people's car parks have gone," he said. "People ask 'will it get any bigger', that sort of talk."

Horne believed the recent heavy rain had contributed to the severity of the sinkhole.

"It's been very wet, and most earth, concrete, things, could have that issue when you've got so much water over all that time," Horne said. "It's going to happen at some point."

Workers at the sinkhole in Parnell, Auckland, that opened up after a main sewer line collapsed. Photo: RNZ / Finn Blackwell

Three-year terms a challenge for infrastructure - Sepuloni

The broken wastewater pipe is part of a sewer which services large parts of central and West Auckland.

Minister for Auckland Carmel Sepuloni said infrastructure in the super city was a major issue. The three-year political cycle made it difficult to shore up long-term infrastructure, she said.

"Not having consensus over political cycles means that it does inhibit us from being able to get ahead with respect to having that infrastructure that we need in place."

Sepuloni said it was one of the areas the country fell short on.

"In some instances, it is the local government's decisions and resourcing that's required, sometimes central government. Particularly for Auckland, infrastructure is a major issue."

Deputy Mayor Desley Simpson earlier the third sinkhole First Up it was a "blimmin' big hole".

"It's almost, dare I say it, in one of the best places it could be, which is in a car park at a no-exit street," she said. "It is still serious."

Simpson said the hole was roughly half the size of a tennis court.

Auckland's bad weather and the city's ageing infrastructure were causing ongoing problems, she said.

"Our ground is very wet, therefore susceptible to this kind of thing."