Contractors have removed soil containing arsenic from a school in the town of Thames.
They are also planning to remove exposed soil from a garden and will try to deal with any similar problems at a nearby pre-school and early learning centre.
The arsenic was found in the suburb of Moanataiari, apparently left over from mine tailings used as landfill 60-90 years ago.
Tests show surface soils had a maximum of 17 times the recommended level of arsenic and the highest deep strata soil had 235 times the recommended level.
A level 10 times that would be lethal.
The Waikato District Health Board is advising residents to wash their hands after handling soil, and not let children play in soil.
It is also advising people not to eat locally grown fruit or vegetables or, if they must, to peel off the outer layer of skin and wash them properly.
About 300 people attended a public meeting in Thames on Tuesday evening on the contamination.
As well dealing with the education sites, contractors will drill bore samples in the gardens of about 200 homes in the suburb.
Arsenic is not used in mining, but is known to occur alongside gold in current or dormant geothermal areas such as the Coromandel peninsula.