A long-awaited bid to re-enter a New Zealand mine where an explosion killed 29 men nearly a decade ago has been suspended at the last moment, dealing another blow to families looking for answers.

A team of experts had been due to enter the drift of the Pike River mine - in the South Island - on Friday to begin a search for evidence into what, in 2010, caused two methane blasts that killed all but two of those inside - including Australians William Joynson, 49 and Joshua Ufer, 25 - and recover what may remain of the men.

But on Thursday, the government abruptly announced the entry had been postponed due to safety concerns, after "unexpected and unexplained" oxygen readings coming from inside.

"The families will be disappointed at another setback, but safety has always been their first priority too," the minister in charge, Andrew Little, said.

Authorities, however, say they will still "definitely" re-enter the coal mine. The timeline is now unclear as the problem is worked through.

The relatives of many of the victims have for years fought for a full search of the site and this week they gathered in the small, nearby town of Greymouth.

Mr Ufer's mother, Joanne, travelled from NSW to be among them.

She said she wouldn't be disheartened by the delay and understood the need for safety to take priority.

"It's not the end. It's as close as we've ever been. It's hard being here and knowing it won't happen. But it will happen. I believe it will happen," she said.

"We've had worse setbacks. This is, in a way, just a blip."

She said she still was grateful to have made the trip and been there with others when the news came through.

Earlier this week, she told AAP that even with the re-entry, it was too soon to get her hopes up about finding her son's remains.

Anna Osborne, who represents a group of the families and whose husband Milton was killed in the explosions, said despite the disappointment, the authorities had made the right called and deserved to be applauded for putting safety first.

"We now that it's not a show-stopper. We know this is still going to go ahead. It's just a delay," she said.

New Zealand Prime Minister Jacinda Ardern will on Thursday night meet with the families.

The relatives will still gather at the mine's entrance on Friday for a ceremony.

Re-opening Pike River was deemed too dangerous by the mine's owner in 2014 but New Zealand's Labour Party in 2017 pledged to make an attempt if elected and set up an independent recovery agency.

While no individuals were prosecuted over the disaster and the mine's former owner - Pike River Coal Limited - was ordered to pay the families of the 29 men and two survivors a total $NZ3.4 million ($A3.2 million) in reparations but went into liquidation.