The Mayor of Thames-Coromandel says he was appalled to learn that the regional council knew as early as 2006 that soil in a subdivision was contaminated with arsenic.
The contamination at the Moanataiari subdivision which has 200 houses was only revealed publicly in August last year.
Dirt tests returned readings of up to 17 times the recommended level - a legacy of the old mining tailings used as landfill.
A 2006 Environment Ministry paper says while staff at the Waikato Regional Council had not done extensive testing of soil under residences in Moanataiari there was potential exposure to a number of contaminants.
The paper noted residents had not been informed of the investigations so the information should be kept confidential.
Mayor Glenn Leach says he can not believe the regional council did not act on the information for five years.
"The first of it that Thames-Coromandel district knew I believe was about April or March 2011 - and that was a staff letter. It just comes through the system. I'd hate to think that there's more of this sort of situation around."
Waikato Regional Council says it didn't tell Thames people about the possibly poisoned soil five years ago because the report that raised the alert belonged to the Ministry for the Environment.
The council also says the national environmental standard for assessing and managing contaminants in soil then was different than it is today.
Paper released under OIA
Thames lawyer and environmentalist Denis Tegg obtained the 2006 Ministry for the Environment paper under the Official Information Act and says he can't believe the report was kept quiet.
"Based on survey information supplied by the regional council, it was ranked the top priority for testing out of 50 other sites nationwide. And yet, here we are five years later before any meaningful testing was done - that's pretty appalling."
The council says the information was not released in 2006 because the national environmental standard for assessing and managing contaminants in soil then was different than it is today.