The Pike River Royal Commission's theory about the cause of the disaster at the West Coast coal mine is being challenged by an electrical company.
The inquiry into the explosions which began on 19 November 2010 and killed 29 men is nearing the end of its third phase in Greymouth.
The commission's working theory relies heavily on evidence given to the Department of Labour by Australian electrical consultant Tony Reczek.
Mr Reczek told the commission on Monday the mine was rife with potential sparks because of overheating cables and unusual power systems.
But the supplier of some of the key equipment, Rockwell Automation, has told the inquiry the findings are simplistic and objected that it has not had the right of reply.
The Royal Commission has agreed the company will provide a full report within a month.