A long-awaited discussion document about opening up conservation land to mining was likely to be released today, Prime Minister John Key said.

First, Cabinet would again discuss the issue, Mr Key told Breakfast on TV One.

"Cabinet is going to go back and discuss the schedule four and the wider discussion document on mining activity in New Zealand and in all probability that will be released today," he said.

Energy Minister Gerry Brownlee last year initiated a stocktake of minerals in the conservation estate, including those parts of it which are in schedule four of the Crown Minerals Act and are protected from mining.

The stocktake estimated the value of the minerals at a conservative $140 billion and Mr Key said "significant" parts could have protection removed.

Since then Forest and Bird said leaked information showed the Government had planned to remove half a million hectares from schedule four but, fearing a public backlash, had trimmed it down to 7000ha.

It said the Government wanted to mine in three national parks, and in Parliament the Greens said a $4 million subsidy was in the pipeline for mining companies to prospect.