A woman whose husband died in the Pike River Mine explosions hardly slept last night because she was so excited about this morning's re-entry into the Pike River Mine.

Photo: Supplied / Pike River Recovery Agency

Anna Osborne, whose husband Milton was among the 29 men killed in the 2010 disaster, was among the families who campaigned for the re-entry.

A three-person team went into the mine and opened the doors to an air lock just inside and checked the condition of the entry tunnel.

They came back out to a big cheer from some of the families gathered at the entrance.

Ms Osborne said she had waited a long time for the re-entry to happen.

"It feels bloody amazing, finally, I've been wanting to see this for a very long time. We've had so many hiccups and roadblocks along the way," she said.

The Pike River Recovery Agency said today's re-entry went according to plan.

Dinghy Pattinson from the agency said everything went "extremely well".

Re-entry has happened! Dinghy, Kirk and Chris opened the doors and went inside, checked the drift condition, and then came back to a big cheer from the families. — Pike River Recovery Agency (@PikeRecovery) May 21, 2019

It's the first step in a $36 million plan to explore the 2.3km-long entry tunnel in the search for remains and clues as what what caused the mine to explode, killing 29 men.

The agency's chief operating officer Dinghy Pattinson was one of those who entered.

Photo: Supplied / Pike River Recovery Agency

This morning Prime Minister Jacinda Ardern said today was significant and would hopefully answer some questions.

"This is only really the beginning. We still don't know how much information, how much evidence, and whether we will be able to make a recovery of those who lie beneath."

The plan to re-enter the access tunnel was called off earlier in the month after high levels of oxygen were detected, making entering potentially unsafe.

The problem was identified as a tube for monitoring equipment that had been leaking into the area, and has since been fixed.