Work has started on the Pike River mine re-entry operation and an air force helicopter is helping remove debris from the top of a ventilation shaft.

It's nearly three years since explosions in the New Zealand coalmine killed 29 men and their families have been trying since then to persuade the government and the mine's owner, Solid Energy, to retrieve their bodies.

A technically feasible plan was worked out earlier this year and the government agreed to put up $NZ10 million ($A8.87 million) so it could go ahead.

The big problem has been safety, because there is still methane gas in the mine.

Re-entry work began on Sunday and the first step is sealing off the ventilation shaft in the main entry tunnel.

When that has been done nitrogen will be pumped into the tunnel to force out the methane, and mine experts will be able to walk down the 2.3km shaft to a rockfall.

Most of the bodies are believed to be in tunnels beyond the rockfall.

"Safety is paramount and the project will be carefully managed with a risk assessment undertaken at each stage," Energy Minister Simon Bridges said on Monday.

Defence Minister Jonathan Coleman says an air force NH90 helicopter, supported by air lift personnel, has removed nearly 25 tonnes of debris from the top of the ventilation shaft.

"The NH90 has twice the lifting capacity of civilian helicopters," he said.

"It is state-of-the-art technology and they expect to transport up to 20 loads this week."

Mr Bridges says at this point the operation doesn't include entering the main mine workings beyond the rockfall.

"The government can't speculate on re-entering the main mine until the tunnel re-entry has been successfully achieved."