The Taranaki Regional Council says the clean-up of contaminated soil at gas well sites in South Taranaki could take up to a year to complete.
Shell Todd Oil Services is several months into cleaning up contaminated ground at Kapuni wells.
The council's director of environment quality, Gary Bedford, says the contamination of soil is the result of 50 years of drilling in the area and the clean-up process is well underway.
Mr Bedford says at least six years ago Shell told the council it had found contamination under so-called flare pits where waste fluids are dealt with.
The pits were also used occasionally for waste storage, which the company was not allowed to do.
Mr Bedford says because the company exposed what was happening, it was not prosecuted.
The council has since increased its monitoring of well sites and requires all pits to be lined to prevent contamination.
Mr Bedford says the public should not be concerned.