WELLINGTON - Two of the world's most toxic former mines are to be cleaned up after decades of leaching poisons into the ground in New Zealand's South Island, the government said on Wednesday.

The two contaminated mines on one site near Reefton, in the West Coast region, would be cleaned up at a cost of 3 million NZ dollars ($2.26 million) in a project jointly funded by the Ministry for the Environment and the Department of Conservation, Environment Minister Nick Smith said.

"The Prohibition and Alexander mine sites are acutely toxic and a blight on New Zealand's clean, green reputation," Smith said.

According to the minister, the levels of arsenic were among the highest recorded in the world at 400,000 parts per million on land, or 500 times the safe level, and in water at 300 parts per million, or 33,000 times the safe limit for drinking water.

"We need to clean up this site so as to prevent ongoing contamination to the surrounding environment and make the site safe for future generations," he said.

The Prohibition mine site had been contaminated from the operation of a roasting plant from 1935 to 1951, when arsenic bearing ore was roasted to release gold.

The sites also had high levels of mercury and cyanide.

The Department of Conservation inherited the site in 1987 and it has been fenced off to prevent public access and harm.

The cleaning up would involve removing arsenic contaminated soil, securing the soil in sealed barrels in a water-tight pit and capping of the surface around the pit and tower, and it would include a water treatment plant to protect surrounding natural water bodies from contaminants.

The contamination was a legacy of inadequate oversight and requirements of previous mining activities in the West Coast, he said.

"We need to repair the environmental damage and clean up this site, but also ensure that we properly regulate mining activities today so as not to create more problems of this sort in the future," Smith said.