Experts says fluctuating gas levels in the Pike River Coal mine indicate there is a fire that could burn for decades.
Pike River Coal chief executive Peter Whittall says analysis from the mine is showing the levels of flammable methane and poisonous carbon monoxide are fluctuating with the outside air pressure.
Mr Whittall says this indicates some heating in the mine but he does not know the precise source.
Canterbury University senior lecturer in engineering geology David Bell says the blast has left a smouldering fire and the future of the mine could depend on whether it is in destroyed equipment or the coal seam.
Queensland University Minerals Industry Safety Centre operations manager David Cliff says a coal seam fire will burn until oxygen is exhausted, either by smothering the coal face or blocking up the mine completely.
Both men say there have been a number of mine fires that have burned for decades.