Health authorities say young children are most at risk from poisoning following the discovery of high arsenic levels in the Thames suburb of Moanataiari.
Tests of surface soils have returned readings up to 17 times the recommended level, a legacy of the old mining tailings upon which the suburb is built.
Deeper soil is even more toxic.
About 300 residents attended a public meeting in Thames on Tuesday night where they were warned to not eat produce from their garden.
They were also told to dust regularly and wash their hands before eating.
A Waikato District Health Board medical officer of health, Dell Hood, told the meeting young children who like to eat soil are the most at risk.
She said although it is unlikely, those children could become acutely sick.
However, Dr Hood says that is a worst case scenario.
Residents were told that all properties will need to be tested and will face remediation works if the soil is toxic.