Families of those who lost their lives in the deadly Whakaari eruption were among those acknowledged at a mihi whakatau/greeting on Monday, to herald the start of a trial over health and safety failings leading up to the disaster.

The legal proceedings are the result of WorkSafe's most extensive and complex investigation to date. It started with 13 defendants - and now there are six.

It has been three-and-a-half years since the disaster on December 9, 2019, which claimed 22 lives and left 25 seriously injured.

The judge-alone trial is due to start tomorrow in Auckland, a day later than planned due to 11th hour guilty pleas by three helicopter tour operators on Friday.

A packed public gallery stood for the opening waiata before Ngati Whatua's Reverend Otene Rewhiti, the first to speak, acknowledged the families in attendance and their loved ones who died.

It was a difficult day for many, bringing back the horror of the disaster that gripped the nation.

The trial starts this week, but since WorkSafe first laid charges against 13 parties in 2020, six organisations have pleaded guilty and another had a charge dismissed.

Whakaari's owners, three members of the Buttle family, are now the main defendants facing trial over health and safety failings in the lead up to the tragedy. That is alongside two companies that booked and marketed tours to cruise ship passengers.

Families of victims and defendants filled the public gallery.

Among those watching the livestream in a virtual courtroom set up at a conference centre in Whakatāne was Judy Turner - the town's mayor at the time of the eruption.

"The interesting one will be for those who have pleaded not guilty to the charges and how that plays itself out. What the implications are for those people, and if those things had been in place, would it have made a difference on the day, I think, is what people are wondering."

Like each hearing in court so far, the trial days will begin with an acknowledgement of lives lost.

Judge Evangelos Thomas will be presiding.

"It is appropriate as we always do to pause, acknowledge, to remember and honour those who perished, to acknowledge and think of those who suffered so greatly, those who continue to suffer so greatly."

The trial starts today with WorkSafe giving its opening address.

The charges do not relate to the rescue and recovery efforts after the eruption.

-RNZ.