The Pike River mine is one of the most difficult an Australian mine safety expert has dealt with.

Efforts are continuing to retrieve the bodies of 29 men killed after a series of explosions in the coal mine, about 50km from Greymouth. The first was on November 19.

David Cliff, an Associate Professor at the University of Queensland, is advising on the Pike River operation which is relying on a jet-propelled machine brought in from Australia to extinguish the fire before rescuers can enter the mine.

Operation of the machine - the Gorniczy Agregat Gasniczy (GAG) - will continue throughout the weekend, as will monitoring of gas levels and temperatures.

Tasman district area commander Superintendent Gary Knowles said Prof Cliff had been back at the mine site today.

"He's described the mine as one of the most challenging he's had to deal with in terms of the terrain, isolation and access," Mr Knowles said.

Once the fire is put out, the GAG machine will blow nitrogen down the shaft to try and cool the mine down.

"Clearly the fire must be extinguished before the next steps can happen in making the mine safe to enter," he said.

Acnowledgements: :**:- NZPA