The Waitemata District Health Board has apologised for accidentally discharging diesel into Lake Pupuke on the North Shore.
People have been told not to fish or swim in the lake, and a small number of birds have died because of the diesel spill from the nearby hospital.
The board told those living around the lake it was sorry, that it accepted responsibility and that it was working to help fix some of the problems.
Richard Hursthouse, a local GP and chair of a branch of Forest and Bird, said earlier that the spill had a severe impact and the board was minimising it.
They were calling it "seepage", when it was most likely a spill of several hundred litres that left a slick across the lake, he said.
He said they should cast their apology much wider than the few hundred people who lived around the lake, because it was used by many more people.
"They need to admit that this is a major event, that it isn't caused by rain and that it's not seepage, it's not relatively minor. It's a major environmental pollution event that should never have happened," he said.
The DHB's head of corporate services, Robert Payne, said it took the incident seriously and the board "absolutely apologises".
The spill was caused by a containment tank that had two holes in it and leaked into the stormwater system.
The DHB had fixed the leaks and was doing everything it could to stop more entering the lake, Mr Payne said.
In a letter to Mr Hursthouse, the DHB said the leak came from its tanks and stopped a week ago.
But diesel continued to wash into the lake through stormwater pipes after heavy rain, it said.
The board said it was working with the Auckland Council, the Department of Conservation and environmental experts, and had removed diesel-affected weed.
Mr Hursthouse said the board must work to ensure a similar spill did not happen again.