New Zealand Prime Minister John Key said on Thursday he did not know when the bodies of 29 miners will be returned to their families.

Key went to South Island West5 Coast town of Greymouth on Thursday and met with the families of the miners to express his condolences.

He promised there would be an inquiry into the disaster. He could not say when the bodies would be returned to the families.

"I don't think it will take days, we are talking longer than that," he said.

New Zealand police said there is no quick fix when it comes to recovering the bodies of the 29 men in the Pike River coal mine.

Gary Knowles, the operation chief, said in Greymouth that it could be days or weeks before the environment in the mine is stabilized.

He said air samples are still being taken and there is still an extreme risk to send people underground.

He confirmed that two Defence robots equipped with cameras were in the mine at the time of the second explosion.

Earlier, Assistant Police Commissioner Grant Nicholls said no decision has been made as to when and how the bodies of the miners will be recovered.

He said that before the bodies can be recovered, the heat source and gases in the mine need to be identified, and the risk of another explosion needs to be assessed.