Auckland Council says the millions of litres of contaminated water that flowed into the Waitematā Harbour during the major SkyCity fire will not cause long term damage.
Arsenic and chromium were among the most concerning contaminants discovered, however tests results - revealed today - confirmed once the water had reached the harbour outfall it had been "heavily diluted".
At the height of the fire that broke out on Tuesday last week in the under-construction New Zealand International Convention Centre firefighters were using 250 litres of water a second.
Watercare confirmed a total of 27 million litres was used to extinguish the blaze, or about one per cent of the city's total water usage in a week.
Auckland Council Safeswim programme manager Nick Vigar said about 20m litres of that flowed into the stormwater system and out into the harbour.
There was also about 1.8m litres of contaminated water pumped out of the convention centre basement into the harbour for about 20 hours from Wednesday evening.
Water testing results showed while there were elevated levels of contaminants, the water was sufficiently diluted by the time it reached the waterfront to not cause long term damage to the harbour ecology.
The contaminants came from burning materials - including bitumen - and later oils and chemicals from mixing with cars in the basement.
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The only potential long-term impact would be on sediment quality, he said.
"It is possibly significant about 10 to 100m from the outfall."
However that sediment was to be dredged anyway in preparation for the America's Cup, and treated on land by turning it into "mudcrete".
Despite water pouring into the harbour on the Tuesday afternoon, Auckland Council only issued a safeswim alert the following morning.
Vigar said they issued the alert as soon as they became aware the contaminated water was entering the harbour.
"Ideally it would be nice to know about it sooner... but it was an emergency situation," he said.
Auckland Council freshwater scientist Dr Coral Grant said they took water samples in drains close to the fire, and samples at the outfall on Gourdie St after it had entered the harbour.
"It showed it was more toxic closer to the site but diluted once it reached the sea."
Toxicology results showed there were "low to moderate toxic effects", from metals like chromium and arsenic on shellfish, but only in the immediate area around the outfall.
Grant said people were already not recommended to gather shellfish in urban areas.
An estimated eight million litres of water had accumulated in the basement of the convention centre during the fire fight, where it rose to 1.5m and submerged about 100 cars.
That water had all since been pumped out, Vigar said.
About 1.8m litres went straight into the stormwater system in a 20 hour period from 9pm last Wednesday, due to concerns from FENZ about a build up of hydrocarbons in the confined space.
Vigar said it was an emergency situation and they needed to remove the water as quickly as possible, which involved pumping it into the stormwater system. There was an emergency provision in the Resource Management Act that covered such actions, he said.
Initially there were concerns about potential contaminants damaging the wastewater network's biological reactors, but Watercare had since confirmed the system could handle the contaminated water.
The pumps were switched over to the wastewater network last Thursday evening.
Concerns were raised at the time of the fire about the contaminated water being pumped straight into the Hauraki Gulf.
Moana Tamaariki-Pohe, deputy chair of the Hauraki Gulf Forum set up to advocate for the marine environment, said it could be "potentially devastating" for sensitive ecosystems.
"The Gulf is already stressed, and adding all of this contamination is hugely devastating. Our stormwater system needs to be prepared to handle this so it doesn't happen again."
Vigar said if a similar fire were to happen again there was "very little" they could do to separate contaminants out of the stormwater system.