The Pike River families have welcomed the planned deadline of a March 2019 re-entry to the mine, saying no "crime scene" should remain uninvestigated for seven years.

Prime Minister Jacinda Ardern and Pike River Minister Andrew Little set out plans for the new Pike River Recovery Agency, including a target of re-entry by March 2019, after Cabinet signed off on it on Monday.

Little said he personally would make the decision on whether to go ahead with a re-entry into the drift of the mine after the agency conducted risk assessments.

The agency would be set up from January and would work closely with the Pike River families on the process.

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Spokesman Bernie Monk said it was a huge step forward. "This is really important to us, but it is also important to all of New Zealand. We should not be a country where a crime-scene goes unexamined for seven years."

Ardern said the goals of re-entry were to recover any remains and collect any evidence for the cause of the blast in 2010 that claimed 29 lives.

"It also has the purpose of giving Pike River families much overdue closure and, if indeed it is possible, peace of mind."

Labour committed to a re-entry as part of its election campaign, but has since said that was not an absolute guarantee.

Ardern said her commitment to the families was to do everything within the Government' power to attempt a re-entry, but safety would be priority.

She said it was possible there was information the Government was not aware of, or expert advice that countered the advice the families had. If that happened, it would work through it with the families.

"There will be risks. Our job is to mitigate them as far as possible and to weigh up whether there is an acceptable level of risk. But as I've said, there were risks every day that those miners walked into that mine.

"The risk they took on was an unacceptable level of risk at the hands of the company they worked for. Now it's incumbent on us to make the right decision to try and re-enter that drift, but we have to do it with all the information.

"Any decision to re-enter will be based on a thorough technical assessment of the risks and advice on how the risks can be mitigated. The families know that we will not endanger any more lives, and that has been one of their most important principles."

Amy Adams, National's spokeswoman for Pike River, said Labour had wound back its initial promise to the families of a re-entry and was now close to that of National's - that safety was paramount.

"The Government is now recognising that it cannot waive health and safety laws, rush or force a re-entry."

She said National had supported a re-entry provided it could be safely achieved. "I would encourage the Government to listen to the experts."

Little said the decision to go ahead would be his but the Pike River Agency would be legally liable for it and mitigating risks before an entry was attempted.

It will be a small Government department and could be based in the South Island, closer to the mine. It will be dissolved once the work was completed or abandoned. It has a budget of about $7.6 million a year.

Family members Monk, Anna Osborne and Sonya Rockhouse will be briefed by officials at the Ministry of Business, Innovation and Employment on Tuesday and will remain involved.

Osborne said the involvement of the families was important and welcome. The families were confident the work could be done safely because its own local and international experts had shown that - but the former National Government had refused to work with them.

She was "immensely pleased" that the new Government had made it such a priority, saying the families had not expected it.