New Zealand has plummeted in international environmental rankings, while a powerful North American conservation group has warned the Prime Minister not to mine national parks.

New Zealand has dropped to 15th on the environmental performance index from Yale and Columbia universities in the United States after being at the top of the index only four years ago.

Meanwhile, Sierra Club International vice-president Richard Cellarius has written to Prime Minister John Key asking that national parks and other protected areas be left alone when the Government considers mining regions.

Sierra, with 1.3 million members, is the oldest and largest environmental group in the US.

In his letter, Cellarius warned that visitors may reconsider coming to New Zealand if the country's beautiful vistas were marred by mining.

"Long-term protection should not be sacrificed for immediate commercial gain."

He was particularly concerned about Mt Aspiring National Park.

"To remove any portion of the park and the World Heritage area for short-term economic gain is an affront to the international community that has worked tirelessly to protect the Earth's wild places."

The Green Party says the Government is considering mining large sections of the Coromandel, as well as parts of Fiordland National Park, Kahurangi National Park, Paparoa National Park and Mt Aspiring National Park.

Green Party co-leader Metiria Turei said the Sierra letter put New Zealand on notice.

"Mining our conservation land will be costly to our tourism industry and export industries that rely on our clean, green brand."

New Zealand Minerals Industry Association chief executive Doug Gordon told TVNZ's Q+A show yesterday that the Government was exploring more mining opportunities to try and "close the GDP gap".

He said tourism and mining could coexist.

The environmental performance index noted New Zealand's "surprising" increased reliance on thermal energy.

The study ranks 163 countries on 25 indicators, ranging from climate change to water and air pollution.

Provisional results place Iceland first, with France and Britain ahead of New Zealand.