Initial monitoring of Auckland's Waitematā Harbour suggests sewage overflows seem to have been localised to a few areas, Watercare said on Friday.
"So, while it is too soon to reach conclusions, what we're seeing so far indicates the impact of the overflows is concentrated on the area in the immediate vicinity of the overflow locations," said environmental manager Nathaniel Wilson.
The most-contaminated spots appear to be from Mechanics Bay to Wynyard Quarter, he said.
He said the nature of the harbour means "there was sufficient dispersion and dilution" away from the sewage discharge point.
"It also appears that most of the solid matter had settled in the pipe and didn't come out at the overflow point" - possibly why locals haven't spotted sewage washing up on beaches.
Staff will continue to test bagged oysters and sea water for contaminants like bacteria, viruses and metals until at least the end of December, Dr Wilson said.
Watercare uses oysters "not to demonstrate that the shellfish is safe to eat", but rather as an indicator of environmental health as they are filter-feeders, he said.
"The good news is these levels dropped significantly when the overflows had stopped."
Warmer weather
Auckland Council has urged the public to keep checking SafeSwim for the best swimming spots as the weather heats up.
Safeswim is showing black pins ('do not swim' alerts) at six locations from Herne Bay to Ōkahu Bay, as they are still contaminated by sewage.
Two weeks ago, Ngāti Whātua Ōrākei began to lift the blanket rāhui over the whole Waitematā but some beaches are still off limits including:
- Ōkahu Bay
- Te Tinana/Wilsons Beach
- Judges Bay
- St Marys Bay; and
- Masefield Beach Reserve at Curran Street.
Black pins remain on Herne Bay and Sentinel Rd Beach in addition to the five under the rāhui.
The sewer sinkhole nightmare
It all comes five weeks after a sinkhole formed in Parnell above the Ōrākei Main Sewer, causing the vital wastewater pipe to become blocked.
As a result, millions of litres of raw human sewage had been flowing into the harbour every day.
Watercare staff installed a temporary pump station in Parnell three weeks ago to divert the sewage which slashed the volume of overflows.
Sharon Danks, Watercare's head of service delivery, said the temporary sewer bypass is holding up well but it's prone to blockages.
"We do have some issues with the pumps blocking from time to time, so we are always urging our customers to only flush the 3Ps - pee, poo and toilet paper."
Watercare staff have partially opened the steel door inside the Ōrākei Main Sewer, which lets a small flow of wastewater through.
"This removes the risk of overflows at times of the day when flows are higher than average and reduces the risk of overflows in wet weather," Danks said.
Some large boulders remain inside the sewer, which Watercare hopes to remove before the year ends.
"The design for the sewer's repair is progressing well," she said, with construction materials anticipated to arrive by the end of December.
Three weeks ago, Danks said repairing a large section of the Ōrākei Sewer was likely to take about five months.
When is it safe to swim?
Despite the temporary sewer fix, overflows will still happen, especially after rainfall, Watercare said.
And, as for when the worst-affected beaches will be safe again for swimming, Brent Gilpin, the Institute of Environmental Science and Research's (ESR) senior science leader, told Newshub it depends on nature doing its thing.
"Sewage not only includes microbial risks of diseases like norovirus, campylobacteriosis and cryptosporidiosis, but also a range of potential chemical and other toxic risks."
"Natural processes degrade many of these and dilution will reduce risks," Gilpin said.
Time taken to degrade depends on the contaminant and weather conditions, he added.
"The more flushing of the harbour the better."
Ongoing water quality testing
Watercare technicians do about 250 water quality tests daily from sites across Tāmaki Makaurau.
Gilpin told Newshub people shouldn't swim after rain or storms, until those tests return to a safe level.
"Some of the contaminants will end up in sediments, which when they get stirred up can mobilise risks into the water column," he added.
He said young children, the elderly and immunocompromised people should wait "weeks rather than days" to swim at the most-contaminated beaches.
Dr Wilson said Watercare is committed to enhancing the wider Waitematā, such as restoring mussel beds or improving areas near discharge points.
"We're following guidance from Ngāti Whātua Ōrākei and hope to have conversations with other organisations that work to protect and restore the harbour."
Gathering kaimoana
Auckland Council and ESR urge people to not collect seafood, especially shellfish, for 28 days at beaches after sewage discharges.
Gilpin said that's due to the risk of getting sick.
"Risks include from the consumption of the seafood itself, particularly if eaten raw, but also from collection of seafood and potential cross contamination from seafood to other foods or people."
In terms of marine life in the worst-affected spots in the Waitematā, Gilpin said nature is resilient.
"So eventually, as long as [we] don't keep having these issues, marine life will recover."
Many places around Aotearoa used to dump raw wastewater straight into rivers and oceans, Gilpin said.
"We no longer consider that as an acceptable solution but, unfortunately, it is something that will happen from time to time, particularly under extreme weather events and infrastructure pressures."
At the start of October, marine scientist Andrew Jeffs told Newshub the sewage was "choking any organisms that'll be filter-feeding", including mussels, oysters and sponges.
"Anything that was living there will be extinguished," Prof Jeffs said.
Marine ecologist Professor Simon Thrush told AM in September marine life will be affected by the sewage, especially in the backwater areas of the Waitematā.
The public can check up-to-date information about the best spots for swimming on Safeswim here.
More public health advice can be found here.
