The gas levels at Pike River coal mine rose sharply overnight after the GAG machine was shut down for repairs and maintenance forcing an evacuation.
The GAG was shut down at 2am today and by 5.30am all staff were evacuated from the hillside because of safety fears around rising gas levels.
By about 7am the situation had stabilised enough to continue maintenance on the GAG.
Operation Commander, Superintendent Gary Knowles said the nitrogen generator had not worked as effectively as had been hoped at neutralising the mine.
Oxygen levels had risen significantly after the GAG shutdown and that raised the very real threat of further fire.
"Until the GAG can be repaired and reinstated the situation at the mine is very unstable," Superintendent Knowles said.
"This highlights the level of risk being faced by the teams working to stabilise the mine. As I have said all along, Police will not endanger any lives in what is clearly a volatile situation."
Early this afternoon the GAG was being prepared to re-start. It is expected to take six to 12 hours to re-pressurise the mine once the GAG is operational.
Meanwhile, Canterbury University Geologist Professor David Bell is at the mine assessing the situation and providing further expert opinion on the best options for managing the situation.