Workers at the Pike River mine were forced to evacuate the hillside early on Monday, because gas levels rose sharply when the GAG machine was shut down for repairs and maintenance.
The machine, which is being used to neutralise the mine's atmosphere, was shut down at 2am. The subsequent sharp jump in oxygen inside the mine raised the risk of another fire and all staff were forced to leave at 5.30am for their own safety.
But they returned just under two hours later to continue maintenance on the GAG, which was eventually restarted at 3.30pm.
The head of the operation, Superintendent Gary Knowles, says the nitrogen generator - which arrived in Greymouth this weekend from Australia - has not been as effective as hoped.