Two dams in the Hunua Ranges which supply about 20 per cent of Auckland's water were closed for four months this year after traces of a toxic herbicide applied to nearby forestry blocks got into the dams' supply lakes.
Last night, Watercare Services said there was no risk to public health.
Auckland Council was told of the incident, but both the council and Watercare Services kept it quiet from the public until a whistleblower contacted the Herald yesterday.
Watercare communications manager John Redwood said the public were not notified because "we were confident there was no risk of water from the lakes being drawn into the supply network at any time".
After a forestry block on council land in the Cosseys, Wairoa and Mangatawhiri dam catchments was sprayed on May 12, Watercare detected traces of the herbicide metsulfuron-methyl in the Cosseys and Wairoa lakes.
Metsulfuron-methyl is a residual toxic herbicide used to kill broadleaf weeds and some annual grasses.
Watercare said the likely cause was run-off from surrounding blocks as a result of unexpected heavy rain two days after spraying. GPS tracking by helicopter confirmed spray was not applied to the lake areas.
Mr Redwood said there was no chance the herbicide got into the water supply because Watercare always had advance notice of spraying, it put extra monitoring in place on May 12, the herbicide was detected at the other end of the lakes from where water is drawn off and there was a rigid testing regime.
Before any potentially toxic water could have been drawn from either lake, they were isolated from the supply network and an intensive sampling programme by Watercare and independent laboratories began.
No presence of the herbicide was detected after July 21 and after independent advice from a UK-based water scientist, Dr Colin Fricker, the lakes were back in service last month.
Watercare said New Zealand drinking standards did not cover metsulfuron-methyl. But Australian guidelines said it would not be a health concern unless the concentration exceeded 0.2mg per litre.
Initial readings in both lakes were well below this level, Watercare said.
In a statement issued by the council this morning, chief executive Stephen Town said he was confident public safety was never at risk but was reviewing the reporting procedures around potential drinking water contamination.
"I am confident that proper safety procedures were in place and were followed by the parties concerned.
"Watercare took all steps required to ensure that neither water quality nor water supply were put at risk.
"There remains a concern about who should have been informed of these events and when, and whether this should have been made public at the time.
"I will therefore undertake a review of council's reporting procedures.
"Our organisation must be as open and transparent as possible ? and that includes matters within the wider Auckland Council family that are in the public interest," he said
Dr Leo Schep, a toxicologist at the National Poisons Centre in Dunedin, said the Australian guidelines were robust, based on scientific data, and he would be happy with the Watercare figures if they were below the recommended guidelines.
He could not comment on the herbicide without looking it up.
Fellow toxicologist Dr Michael Beasley said the risk of getting sick from the herbicide was very low given the reported concentration level. "It's very likely that the risk is negligible and I think it's very low."
The Hunua Ranges have five dams, supplying about 65 per cent of Auckland's water. The Cosseys and Wairoa dams account for about a third of the water in the ranges, or about 20 per cent of the city's needs.
The water from each dam is piped to the Ardmore water treatment plant. It is then stored and transported through pipes up to 1.9m in diameter into reservoirs nearer the city.
The other dams in the Hunua Ranges are the Mangatangi, Mangatawhiri and Hays Creek.
Auckland Council last week voted to switch to 1080 for pest control in the Hunua Ranges Regional Park. It says stringent controls will ensure the poison does not enter water supplies.
Auckland Council has limited its comment to saying that as landowner it was advised by Watercare of the issues.
Two councillors, Mike Lee and Cathy Casey, have criticised the council for not informing them or the public about the issue.
Mr Lee said: "Is there something in the water causing the sick culture of managerial secrecy in the Auckland Council? Last week council management persuaded the Councillors to approve the aerial dumping of 1080 over the Hunua water catchment. While management were briefed by Watercare on 23 June they never disclosed this to the Councillors nor the public."
Ms Casey said it was unbelievable that Watercare's closure of two dams in the Hunua Ranges for four months due to a herbicide contamination was not disclosed to councillors ahead of - or during - the 1080 debate when questions were asked about the safety of Auckland's water supply.
What role do the Hunuas play in supplying Auckland's water?
They have five dams, supplying about 65 per cent of the city's water. The Cosseys and Wairoa dams hold about a third of the water in the ranges - about 20 per cent of the city's needs. Their water is piped to the Ardmore treatment plant.
Was there a health risk?
Watercare says public safety was not compromised at any point.
What is metsulfuron-methyl?
It is a residual herbicide used to kill weeds and some grasses.