The Council of Trade Unions has started court action in a bid to bring former Pike River Mine manager Peter Whittall to trial.
Shortly before Christmas, all 12 charges against Mr Whittall over the tragedy which killed 29 men were dropped.
The CTU said it would consider a judicial review of that decision.
President Helen Kelly told the Greymouth Star it had filed preliminary proceedings with the Wellington District Court on December 24, seeking more information.
Ms Kelly said it was filed on behalf of a sample of the families, and the first step was to get all the information.
"We know there's broad support."
The courts had been shut over the holiday period, but the union had followed it up last week.
"Hopefully we will get something quite soon. It (case) is very, very important."
A separate bid by Wellington accountant Graham McCready to file a single, representative count of manslaughter is on hold until later in the year at the request of the Pike River families.
The charges, dropped in mid-December, had been laid by the Ministry of Business Innovation and Employment under the Health and Safety in Employment Act.
However, the Crown said that after an extensive review it was "not appropriate to continue with the prosecution against Mr Whittall".
Instead, Mr Whittall and Pike River Coal Ltd (in receivership) offered a voluntary payment on behalf of the directors and officers of the company to the families of the men killed in the mine, and the two survivors.
Judge Jane Farish told the court the likelihood of a prosecution was "extremely low" and the case may never have reached trial.