Tomorrow's planned Pike River re-entry has been delayed after "unexpected and unexplained" atmospheric readings in the mine.
Minister responsible for Pike River re-entry Andrew Little said there were elevated oxygen levels that needed to be explained and the recovery efforts were taking a "safety first" approach.
"Yesterday unexpected and unexplained readings were reported by the atmospheric monitoring systems in the Pike River mine, leading to re-entry operations being suspended," Mr Little said.
"The safety first approach means that if we can't explain that change in atmostpheric conditions then we suspend operations," he said.
He did not have an estimated time of when the re-entry may happen, and did not want to make any predictions.
"It could be days, it could be weeks."
"Safety has always been our first priority, and will continue to be. In these circumstances the appropriate precaution is to temporarily suspend operations.
"The Prime Minister and I will join the families on the West Coast tonight, and families will receive a comprehensive briefing from the Agency tomorrow," Mr Little said.
"I have spoken with Anna Osborne from the Pike River Families Reference Group. The families will be disappointed at another setback, but safety has always been their first priority too.
"They've been involved in the decision making in this project all the way through. They are certainly keen to see the mine site, see the preparations for the project. This is a setback, it's a temporary setback but we are still planning for and intend to proceed with the project."
"The families are gathering in Greymouth and they'll receive a full technical briefing tomorrow," Mr Little said.
"They'll see the lay of the land, the equipment is being used, but it will I expect it will take us some days as we gather the expert evidence to understand what is happening and for the Agency then to respond and and mitigate the issues that have arisen."
Mr Little told reporters that the readings of elevated oxygen levels were at the borehole, near the rockfall.
The increased levels had not been predicted, and experts advising the government could not explain it, he said.
"When you've got a methane-producing environment as you do there - it's the mix of methane and oxygen that makes it dangerous - that dictates what you do to ventilate the atmosphere, so it is really about making sure that the way we respond through ventilation is going to make sure that the atmosphere continues to be respirable and there is no risk of volatility or explosion."
There would be a range of possibilities in working to explain the readings - including that the monitoring equipment isn't accurate - and all would have to be explored, he said.
"We've got a discount that possibility, it may well be that oxygen is coming in through the strata, we need to we need to confirm or clarify or discount that possibility. There will be a range of possibilities. Each needs to be explored so that we know how to respond."
"I back the Pike River Recovery Agency to take the time needed to fully understand the cause and significance of these new readings.
Further testing and investigative work will be completed over the next week and a meeting of ventilation experts will then convene later in the month.
Pike River Recovery Agnecy head Dave Gawn told media in Greymouth the delay was due to an unexpected reading in the gas monitoring.
He said safety was key.
"Dinghy and I would have no hesitation calling stop to the proceedings," he said.
Dinghy Pattinson - the Pike River Recovery Agencies chief operating officer - said two days ago the agency started a breaching operation of a concrete seal at the start of the mine.
But at the same time they found unusual gas readings at a borehole monitoring point (at borehole 51) deeper in the mine, above rockfall.
He did not know what caused the reading, and had not seen it before.
The cause of it could be something as simple as a damaged tube, he said.
To figure out the cause the agency had lowered another gas detection tube down the same borehole.
But Mr Pattinson said the re-entry would definitely happen, he just could not say when.