Regional councils admit the nearly 20,000 sites nationwide confirmed as contaminated land could be just a fraction of the real number.
A report released this week by the Ministry for the Environment, Our Land 2018, showed the overall extent of land contamination was unknown, but 19,568 sites have been confirmed as contaminated.
Regional councils keep records of specific sites where contamination is confirmed from pesticides, timber treatment chemicals, mining run-off, fire-fighting foam and other hazardous substances.
But the report said many regional councils estimated that up to three times as many sites could be identified in their areas with "further work".
It noted there was currently no integrated overall dataset showing the extent of confirmed contaminated sites across the country.
"We know what kind of land contamination can happen in New Zealand and how these chemicals may impact on human and environmental health but we are unable to report on the overall extent of land contamination," it said.
The chairman of the Environmental Defence Society, Gary Taylor, said the report showed a major overhaul of the resource management system was needed.
Too often regional councils were highjacked by vested interests and conflicts of interest, he said.
"We need to look at a whole range of things, not just the RMA [Resource Management Act], but the agencies that administer it, and free them up from conflicts and perverse pressures that can produce bad outcomes."
The lack of a standardised, integrated reporting system meant no-one knew how bad the contamination problem was, he said.
"This report is honest about the gaps. You can't manage what you don't know about."