One of the owners of Whakaari / White Island says in an audio interview played in court that whether their company owed a duty to the victims of the eruption in 2019 was a "hard question to answer".

Andrew, Peter and James Buttle along with their company Whakaari Management Limited (WML) are on trial for allegedly breaching health and safety rules in the lead-up to the eruption on December 9 2019 that killed 22 people.

In an interview with WorkSafe from 2020 played in the Auckland District Court on Friday, WorkSafe inspector Christian Bell asked the brothers whether WML believed they owed a duty to the victims.

Peter Buttle says that was difficult to answer.

"It's an incredibly hard question to answer, because the scale of it was so horrific and it's had such a wide impact on the ... people who died and survivors and their families.

"That it's, it's just, just unimaginable, and to know how to respond to that is just, it's just ... I don't know how to put it into words," he said.

Peter added, "there is no way you can respond sensibly to the scale of that disaster".

Andrew Buttle also says it was hard to answer.

"I mean it's difficult with investigations, coroner investigation, WorkSafe investigation, and ... we're incredibly mindful of the tragedy and its effects. It's impacted us a lot in that regard," he said.

In a statement from Peter Buttle read out by one of the prosecution's lawyers in court, he says they relied on GNS and other groups to monitor the risks.

"We're not volcanic experts, and we have no ability to monitor the islands on a daily basis. That's why it really has to devolve to the operators and GNS and specialists, and the other people that have the responsibility for managing that risk in terms of working out what is safe and what isn't."

He says what happened was a shock to everybody, but it was hard to foresee.

"It's easy to look in hindsight, and I wish we knew, and I wish we had the ability to have a bit of foresight, because what happened was a terrible disaster," the statement read.

In the statement, Peter Buttle says they believed they had put in place everything that they "could possibly put in place" to ensure the operators operated safely, and that they had confidence in the operators.

Earlier in the trial, the court had heard that WML was earning about $1 million a year from charging tour operators an annual licensing fee, as well as a fee for each individual visiting the island.

WorkSafe's prosecutor had argued that the Buttles and WML had failed to exercise due diligence and to invest some of their profits in addressing health and safety concerns - such as repairing the jetty, the only means of evacuation from the island.

She also says WML was aware they could hire GNS Science to perform an assessment, but were unwilling to pay the fee.

Meanwhile, WorkSafe called its last witness for the trial on Friday - the second person in charge of the Whakaari eruption investigation, investigations manager Casey Broad.

Broad oversaw 23 WorkSafe inspectors who worked on the Whakaari investigation, and reported to WorkSafe national manager of investigations Hayden Mander.

Under cross-examination, Broad was quizzed about an emergency response plan developed by Emergency Management Bay of Plenty (EMBOP) relating to Whakaari.

Peter Buttle had said in his statement read out that the plan came out of a meeting they had in 2018 with GNS, police, and Bay of Plenty Tourism and EMBOP to discuss how to make it a safe tourism operation.

The plan's latest version had been shared in September 2019, and had also been sent to two WorkSafe inspectors.

When asked by defence lawyer James Cairney whether WorkSafe had considered turning the lens on itself when it comes to its role in risk management on Whakaari with respect to the response plan, Broad says no.

However, Broad says he had considered an investigation of WorkSafe itself around its role in overseeing adventure tourism activities happening on the island.

Broad saiys he had raised it with Mander, but did not know where Mander had taken it.

"I had suggested to the officer in charge early in the investigation, once we started collating documents, that I believe that would've been a good idea to have an independent investigation running at the same time," he says.

Broad says WorkSafe authorised a third-party to audit adventure tourism operators, and that once they passed the audit, operators would go on WorkSafe's register of PCBUs.

Lucy Xia/RNZ