The Transport Agency knew that Auckland Harbour Bridge operations had contaminated land next to houses at above permitted levels, but decided not to tell local people.
It found high and rising levels of lead, zinc and a cancer-causing hydrocarbon at the bridge's north end in 2010.
The agency's national environment manager at the time Carl Reller says it was covered up.
Reller, who covered major highway projects from 2007 until he resigned in 2014, says the case shows the agency's tendency to disregard the public's right to know when it is in the gun.
The Transport Agency says that Auckland Council, as the regulator, determines whether the public should be told about contamination.
However, all of the documents from the time presume it is the agency's responsibility to go public.
The council has been approached for comment.
