Conservation land totalling 3000 square kilometres will be given over to platinum and gold mining companies for prospecting, the Government has announced.

The South Island land includes the Upper Buller Gorge Scenic Reserve and areas on the Shenandoah and Pelorus Rivers. It would also allow exploration on 240 hectares of the Mokihinui Forks, home to a unique old beech forest.

It includes places which environmentalists battled to protect from logging, such as Southland's Longwood Forest and West Bank Maruia and Granville Forest on the West Coast.

It does not include "high conservation value" Schedule 4 land which cannot be mined.

Earlier this month the Ministry for Business, Innovation and Employment opened a tender for five areas of the South Island and hopes to grant five-year exploration permits next year.

The block offer is called Platinum New Zealand, as the Government wants to entice companies with expertise in metallic metals mining to underexplored areas. World demand for platinum is likely to increase because of supply constraints.

The permits would also allow for gold prospecting, although the gold price has plummeted recently. Three-quarters of the area is on conservation land and environmentalists are alarmed at the potential environmental impacts of metal mining, which produces large amounts of tailings (crushed rock). Metals can be mined using open pit or underground methods, depending on location.

Green Party MP Catherine Delahunty said there was also a risk of the release of cancer-causing dioxins in extraction. "The Government will say it's just exploration, but what is the point of opening it up to exploration if you don't intend to allow mining.

"This is just a tendering process, but it is not reassuring. The depressing thing is that so many of these places, particularly the West Coast beech forests, were fought for to stop logging. Now it is going to be opened up to mining."

Forest and Bird North Island conservation manager Mark Bellingham has spoken out against a gold and silver block offer around the Bay of Plenty and Waikato, which opened in August, and which included conservation estate.

Gold is dissolved in cyanide, and other "nasties" such as lead, cadmium and mercury are often released, contaminating waterways, he said. Mines often go bust and the taxpayer is left with a clean-up bill. "The news is not good on these things."

Waikato University environmental chemist Adam Hartland has spent some time looking at the defunct Tui Mine, which he said was the "best example of how not to do it."

Remediation of the abandoned site, on the slopes of Mount Te Aroha, has cost taxpayers more than $20 million.

"The tailings were left to the elements . . . it's very reactive and it generates acidity," Hartland said.

Heavy metals become more soluble with increased acidity and enter the water, affecting wild and plant life.

"Metal toxicity is the biggest issue," he said. If modern mining is done well "then there is no reason to be concerned about it. If it is done badly, that is the problem," he said.

Almost three years ago Parliamentary Commissioner for the Environment Jan Wright wrote a report on mining on conservation land. She criticised a law change which means conservation and energy ministers have joint sign-off on access. Few of her recommendations have been acted on.

A spokesman for MBIE said the areas were selected "purely because the current data identifies those areas as having a high likelihood of prospectivity."

Only only a small percentage of exploration permits are progressed to mining permits and resource consent process weighs potential benefits for the community, such as jobs, against potential environmental impacts.

THE TENDER AREAS

Longwood (Southland) – four defined blocks totalling 333.40 square kilometres

Grey River (West Coast) – 419.26sq km

Murchison West (West Coast) – 730.89 sq km

Murchison East (Tasman) – 629.98 sq km

East Nelson (Nelson-Marlborough) – 1700.88sq km