The public did not need to know that herbicides leeched into dams which supply 20 percent of Auckland's water, the organisation which manages region's water supply says.
Two dams in the Hunua Ranges were closed from May to September this year after traces of a toxic herbicide were found in the dams' supply lakes.
The herbicide had been sprayed on nearby forestry blocks, and after unexpected heavy rain, had made its way into the dams.
Watercare Services notified the Auckland Council, but the public was not informed of the incident.
Watercare spokesperson John Redwood stands by the decision. "There was absolutely no risk to public health at any time," he said.
But Auckland councillor Cathy Casey said she was appalled by the decision to keep it quiet.
"I am very disappointed. In fact, I'm outraged that I, as an Auckland councillor, did not know that in May two of our dams in the Hunua Ranges were closed due to a herbicide contamination.
"They were closed for four months. Why do I not know that? That's the question I have today asked the chief executive."
Council orders review
The council's chief executive, Stephen Town, said the council would review the reporting procedures around potential drinking water contamination as a result of concern about who should have been informed of the contamination, and whether it should have been made public.
"The moment you describe the event, most lay people say, 'Yes I think I should know about that'. And that's the bit we are going to reflect on and see if we can make some improvements to make sure that trust and confidence amongst Aucklanders is very strong."
Mr Town says the council wants to be as transparent as possible. He said he was confident public safety was never at risk during the Hunua Ranges contamination and all safety procedures were in place.