Police have today advised the families of the 29 Pike River mine explosion victims that planning is under way to drill a further 10 boreholes - months after the years-long operation was officially concluded.

The decision to undertake further drilling at the mine comes after police consultation with an expert mine panel as part of the ongoing criminal investigation into the first explosion at the infamous West Coat coalmine on November 19, 2010.

Detective Superintendent Peter Read said the families spoken to today were surprised by the decision to undertake further borehole drilling but were grateful for the opportunity it offers in terms of evidence gathering.

Some families who have long-campaigned to retrieve the dead men trapped deep underground and also bring criminal charges remain hopeful that a prosecution is still possible.

"The previous borehole drilling operation, which ran from June 2021 to March 2022, provided Police with valuable information to inform our investigation into the underground activity that led to the first explosion at the mine," Read said.

Smoke billows from the ventilation shaft after the fourth explosion in the Pike River coal mine where 29 miners and contractors are still fatally trapped to this day. Photo / File

It comes after police revealed in March this year that the borehole drilling operation, which started in early 2019, had been wrapped up.

Eight sets of human remains were captured in imaging taken during borehole drilling.

Over the past four months, the police investigation team and the expert mine panel have been reviewing the gathered evidence.

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Detective Superintendent Peter Read at November's press conference in Christchurch announcing bodies had been found at Pike River. Photo / George Heard

Police now say the review has established that further borehole drilling is imperative to ensure the investigation team has all the necessary information to reach a definitive conclusion as to what led to the first explosion.

"The families of the 29 men lost at the mine have been waiting a long time for answers, and I'm very aware that in some respects this additional drilling operation could be seen as prolonging that wait," Read said.

"However, I hope the decision also provides some reassurance to the families that police remain absolutely committed to finding out as much as humanly possible about what led to the first explosion at the mine."

Police say they have had "preliminary conversations" with the Department of Conservation and are working through a formal consent process.

Pike River Mine family members Anna Osborne, left, and Sonya Rockhouse are among those still pushing for a criminal prosecution. Photo / NZME

Minister Responsible for Pike River Re-entry Andrew Little, in November, would not be drawn on whether a prosecution was imminent with the police investigation ongoing.

The Pike River Recovery Agency (PRRA) completed its $50m re-entry of the mine's access tunnel to try to recover remains and find any forensic clues last year.

It had been due to permanently seal the mine while police were partway through their borehole investigations.

But some Pike River families who lost loved ones, and had fought for years to try to get authorities to try and find their bodies, launched legal action to try to stop it from happening.

On Friday, November 19, 2010, about 3.44pm, an explosion ripped through the Pike River underground coal mine, followed by subsequent explosions. Two men made it out alive but another 29 were unaccounted for.

The Royal Commission on the Pike River Coal Mine tragedy found that the "immediate cause of the first explosion was the ignition of a substantial volume of methane gas", but could only speculate on what might have triggered ignition.

WorkSafe laid charges against former Pike River boss Peter Whittall in 2013, but the case was dropped after a $3.4 million settlement was paid – a deal the Supreme Court later said was unlawful.