The Department of Labour is prosecuting three unnamed parties over alleged multiple health and safety failures at Pike River coal mine, where 29 men died last November.

It refused to name the accused, saying they had either existing name suppression or had the right to apply for it.

However, it acknowledged there was high public interest in who was charged and was taking "urgent steps'' to ensure their names could be published.

It also declined to detail the charges, which were laid this morning in Greymouth District Court. Each carried a maximum penalty of $250,000.

"Identifying the specific charges could also lead to identification of the three parties charged so the department is unable to specify the detail of the charges,'' it said in a statement released this afternoon.

Last November 19, 29 men miners and contractors were killed in a fatal explosion in the underground mine deep in the Paparoa Ranges.

The dead men's families were treating the move with caution, their lawyer, Nicholas Davidson QC, told The Press today. "I don't think there will be any sense of satisfaction."

The department told him the decision prior to releasing it publicly so he could break the news to the families of the men.

They wanted to know what charges were laid because it would highlight where alleged breaches of law occurred, which mattered than who was charged, Davidson said.

"It tells them at least the prosecuting authority thinks the law has been broken."

The department said its investigation was its most complex it had undertaken and had a team of 15 people involved at its peak with more than 200 interviews conducted.

It sought an extension in April and had until this November 19 to decide whether to lay charges.

Police were also investigating whether to lay criminal charges but were yet to make a decision and had no time constraints.

Evidence had been gathered and was being analysed over whether any charges would be laid, a police spokesman said today.

The Labour Department's oversight of Pike River mine would be closely scrutinised when the Royal Commission into the tragedy resumes at Greymouth District Court next Monday.

The two mines inspectors who monitored the mine, Michael Firmin and Kevin Poynter, would be the first of nine witnesses to appear at the inquiry's phase three over the coming two weeks.

As a result of the department's independent review into its handling of Pike, Labour Minister Kate Wilkinson announced in August it would introduce a new high hazards unit to more closely monitor mining and petroleum production industries.

"The Pike River tragedy was certainly a catalyst for the department to take a close look at what resources and expertise it has available to the mining sector,'' she said at the time.

The department faced criticism during the inquiry's first phase in July for only having two mining inspectors for New Zealand, which was far fewer than under the previous mines inspectorate.

However, Poynter resigned only weeks before the Royal Commission began, leaving Firmin as the sole mines inspector.

The new unit would include three inspectors and a chief inspector for each industry, along with three further administrative and research support staff, costing $1.5 million a year.

Today, the department confirmed a second mines inspector had since been employed and was actively recruiting for the third inspector plus chief inspector.

It only had one petroleum inspector but was also trying to fill the other roles.

The new unit was expected start by next year.