Energy and Resources Minister Gerry Brownlee has refused to rule out open-pit mining in our highest value conservation parks ahead of a review due out shortly.

The Government is considering allowing access to minerals in parts of the conservation estate now off-limits to mining under Schedule 4 of the Crown Minerals Act.

The areas are listed in Schedule 4 for their ecological and scenic values, and include all 14 national parks and the Coromandel Forest Park.

Mining companies have been urging the Government to relax access rules so they can re-examine known gold deposits in the Coromandel and minerals such as tungsten, tin and copper in South Island national parks including Kahurangi and Mt Aspiring.

Mr Brownlee has questioned whether all land in schedule 4 merits highest-value conservation status, and Prime Minister John Key and Conservation Minister Tim Groser have talked of the potential for "surgical" and "discrete" mining in these areas. Activity could take place underground through narrow entrance tunnels to minimise surface disturbance.

But conservationists say many sought-after minerals are in low concentrations and widely dispersed, so large areas would need to be dug up for mining to be economic.

A discussion document on the review has twice been delayed. But after the Cabinet considered the paper on Monday, Mr Key said he was not backing away from the proposal in the face of public opposition.

"We are reflecting on what we think is right and what is achievable."

Asked by the Herald for an assurance that open-cast methods would be ruled out, Mr Brownlee yesterday declined to give it.

"There is a Cabinet decision process in train and I will not be commenting ... until a decision is made and the discussion paper is released for public consultation."