Auckland Council will review its reporting procedures following criticism over a decision not to tell Aucklanders two water supply dams had been contaminated with herbicide.
Two dams in the Hunua Ranges, the Cosseys and the Wairoa, were shut for four months from May after traces of the toxic herbicide metsulfuron-methyl were found.
Metsuluron-methyl is used for controlling scrub weeds and many herbaceous broadleaf weeds.
The chemical had been sprayed on neighbouring forestry blocks but washed into the dams in heavy rain two days later.
The closures only became public following a leak to the media.
Watercare has defended its decision not to inform Auckland residents of the step, with spokesman John Redwood saying that it was unnecessary as there was "absolutely no risk to public health".
Auckland Council chief executive Stephen Town said he was confident Watercare took all steps required to ensure that neither water quality nor water supply was put at risk.
However, he acknowledged there remained "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," he said in a statement released today.
"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."
There five dams in the Hunua Ranges which together supply 65 per cent of Auckland's water.
The two affected dams contribute about 20 per cent.
Both were back in service from September after tests showed water quality had returned to normal.