A fight over charges against former Pike River boss Peter Whittall being dropped has reached the Supreme Court.

Tomorrow, lawyers for the families of two of the 29 men killed in the Pike River disaster in 2010 will argue that Worksafe charges against Whittall should never have been dismissed by the court after families were paid $3.41 million.

He originally faced 12 charges from the Health and Safety watchdog, but the charges were dropped and a deal was cut for Whittall to pay $3.41 million to the victims' families and two survivors.

"We thought that was an unjust decision and we thought that that was blood money and you can't buy your way out of justice in New Zealand, or so we thought," said Pike River family spokeswoman Anna Osborne today.

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Two families applied to the High Court for a judicial review and lost the case.

They appealed that, and the Court of Appeal also dismissed the case, saying the decision to drop the charges was lawfully made.

"We're going to the Supreme Court where our lawyers will be representing us ... hopefully we will get a favourable outcome," Osborne said.

She said they were looking for a "moral victory". They wanted it to be shown that the decision to drop charges was unjust.

"Twenty-nine men don't get killed in the workplace and no one is held to account over it ... it's very much the principle of it all."

Osborne said they were feeling "very positive" ahead of the hearing.

"It's been a long time, actually, a long road we've had to travel to get here, and tomorrow we're hoping it's going to be a perfect outcome for us."

In its decision earlier this year, the Court of Appeal said Worksafe was entitled to consider a reparation payment as one factor in deciding whether or not to pursue prosecution further.

Even if the decision had been shown to be unlawful, it would not have been set aside as payment had already been made to the families.

The charges were dismissed by Judge Jane Farish in the Christchurch District Court in 2013 when the Crown said after an extensive review it was "not appropriate to continue with the prosecution against Mr Whittall''.

Instead, Whittall and Pike River Coal offered a voluntary payment on behalf of the directors and officers of the company to the families of the men and two survivors.

Judge Farish made it clear the surprise turn of events was not a case of a chief executive "buying his way out of a prosecution''.

She told the court the likelihood of a prosecution was "extremely low'' and the case may never have reached trial.