The White Island eruption killed 22 people. Now, the brothers who own the volcano are facing charges.

47 people were on New Zealand's White Island when the volcano erupted on December 9, 2019. 

That day, most of those visiting the island, also known as Whakaari, were taking part in a day trip for passengers on the Ovation of the Seas cruise ship when the volcano erupted after 2pm, sending a plume of ash, gas and rocks into the air. 

The eruption claimed the lives of 22 people, including 14 Australians, five Americans, two New Zealanders and one German. 

A further 25 people suffered serious injuries and burns. 

Now, over three years on, the island's owners, brothers Andrew, James and Peter Buttle, and their company Whakaari Management Ltd, and tour operators I.D. Tours NZ Ltd and Tauranga Tourism Services Ltd, are on trial for allegedly failing to 'adequately protect' tourists and staff.

Each of the companies faces a maximum fine of NZ$1.5 million ($A1.4 million), while each of the brothers charged faces a maximum fine of NZ$300,000 ($A277,000).

Three helicopter tour operators pleaded guilty last week to safety breaches and avoided the judge-only trial, which is expected last 16 weeks. 

During the trial, Auckland District Court is expected to hear from US couple Matt Urey and his wife Lauren Barham from Virginia, who were on their honeymoon when the volcano erupted. 

The pair survived with severe burns. 

British helicopter pilot Brian de Pauw and Australian tourist Annie Lu are also among those on the witness list.

Here's everything you need to know as the trial gets underway. 

How did three brothers come to own an island?

White Island has been privately owned by the Buttle family for almost 100 years. 

Auckland stockbroker George Buttle bought the island back in 1936 and decided not to develop it. George later rejected an approach by the New Zealand government to buy the island in 1953.

The title was eventually transferred to Whakaari Trustee in 2012, which lists George's three grandsons Peter, James and Andrew Buttle, as shareholders. 

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What have we learned from the trial so far? 

On Tuesday, Prosecutor Kristy McDonald told the court the 20 tourists and two tour guides who died in the eruption were given no warning of the risks.

"They were not given the opportunity to make any informed decision about whether they wanted to take the risk of walking into the crater of an active and unpredictable volcano that had erupted as recently as 2016," she said.

"The business of tourism on Whakaari was a risky business. It involved tours to an active volcano, taking people to the heart of the crater in circumstances where no one could predict when an eruption might occur, and if an eruption did occur, those on Whakaari were likely to die or suffer very serious injury... And tragically, that risk was realised."

McDonald said Whakaari Management Ltd (WHL) failed to understand the risk, failed to consult with tour operators on the hazards, failed to ensure appropriate personal protective equipment was provided to tourists and staff and failed to provide an adequate means of evacuation.

Instead, the company left tour operators to monitor the changing risk.

She added that a 2016 eruption of the volcano should have prompted the owner to review the risk assessment.

After the 2016 eruption, NZ geology agency GNS Science banned staff from visiting the crater floor until further notice because of the "heightened state of volcanic unrest", McDonald said.

Despite knowing this, several operators continued taking tourists to the crater from the day after the eruption, she said.

WHL, which made a profit of $NZ1 million ($A930,000) a year from tourists, could have paid GNS for a formal risk assessment but did not, she said.

She blamed the Buttle brothers for the WML's failure to assess the volcano danger.

"The Buttles knew they could obtain expert advice from GNS for a fee," McDonald said. "They chose not to."

I.D. Tours NZ and Tauranga Services also failed to ensure 38 passengers had been properly warned of the risk, McDonald said. Those 38 people "did not receive any health and safety information about volcanic activity or volcanic risk prior to the tour", she said.

The trial continues. 

- With AAP. 

Feature Image: Phil Walter/Getty.