The Government is defending its choice not to disclose the location of places suspected of being contaminated by highly toxic chemicals.
An Environment Ministry investigation of some current or former sawmills where the agent pentachlorophenol was used found the level of dioxin contamination was about three times greater than estimates in the late 1990s.
PCP was used to treat timber until 1988, when the toxin was withdrawn.
One hundred of the 255 sites in question are no longer sawmills and are zoned for commercial, industrial and residential use.
The ministry's local government group general manager, Sue Powell, says there is a record of where all the former sawmill sites are.
However, says there are privacy issues in naming sites that are merely suspected of being contaminated.
The Green Party on Friday renewed calls for a national register for sites contaminated by toxic chemicals, saying New Zealanders have a right to know where they are.
Green co-leader Russel Norman says there are thousands of such sites throughout the country, and many are undocumented.