A law expert has called the Whakaari White Island trial a "messy process" and questioned what might have been the outcome if corporate manslaughter had been on the legislative books.

The island's owner, Whakaari Management Limited, has been found guilty on one health and safety charge brought by WorkSafe, this followed WorkSafe charging 13 parties in 2020.

Six pleaded guilty ahead of the trial and another six had charges dismissed.

It was the ending of a lengthy trial that first began in July, law expert Chris Gallivan told The Mike Hosking Breakfast he didn't enjoy what he'd witnessed. 

"I don't think anybody did," he said in an interview on Monday. 

"I started to say this at the outset but if we had corporate manslaughter on the legislation books then the whole thing might have played out a lot more seriously - if that's the right word - a lot more significantly right from the start."

Gallivan said the non-existence of corporate manslaughter as a charge is a glaring hole in the country's judiciary system. In White Island's case, there had been calls from trade unions for legislation to pass on it. 

It's a charge that, had it been introduced, might have been laid against some of the defendants. 

"It should change, absolutely it should change," said Gallivan when asked if legislation needed to accommodate corporate manslaughter. 

He said New Zealand's laws which cover murder and manslaughter are "a bit of a dog's breakfast" and, while the world demonstrates options like degree of murder, New Zealand's homicide laws are quite complex. 

"It's spread over a number of provisions of the Crimes Act... [corporate manslaughter] would cover a whole lot of people who died at work, as well as the members of the public who died in this tragic situation."

Hosking asked if Gallivan could speculate on how many pleaded guilty because they genuinely were, compared to those who simply saw it as the simplest route to "pay the fine and move on". 

Gallivan praised the question and explained that ten years ago, within the Solicitor General's guidelines on negotiation within prosecution, there was nothing provided on plea deals.

"Over the last ten years, it softened and then went to the stage where the Crown could discuss a plea deal but couldn't instigate it," he said.

"And now it's all gloves off and we, the Crown and defence more than anybody, welcome the plea deals."

Gallivan said he takes "no issues with plea deals per sey", but explained they only work with great oversight from the judiciary. 

The flip side to being careless with plea deals was the possibility of New Zealand's legal system reflecting that seen in the United States, with incremental overcharging beginning to take form. 

"[It] scares the people into pleading guilty to something less," said Gallivan. 

"I don't know about the context of this case, but I do know with plea deals the danger is that you get a creep in prosecution overcharging, on the basis that they know a plea deal will bring it back to something they originally thought was where they wanted it to land."

Whakaari Management and the six other guilty parties will be sentenced in February next year.

Judge Evangelos Thomas said sentencing could take up to two weeks.