The company managing Whakaari/White Island has been convicted for one of the two charges it faced for health and safety failings in the lead-up to a fatal eruption in 2019.
It was the culmination of a lengthy trial, which started in July.
WorkSafe initially charged 13 parties following its investigation of the catastrophic eruption, which killed 22 people and injured 25 others.
This included Whakaari Management Limited (WML), which WorkSafe alleged had breached sections 36 and 37 of the Health and Safety at Work Act 2015.
The individuals who own Whakaari Management and the island itself - Andrew, James and Peter Buttle - had their charges dismissed in September.
WML's defence lawyer, James Cairney, argued during his closing statements last month that Whakaari Management was nothing more than a landowner.
Judge Evangelos Thomas disagreed.
"WorkSafe has established that Whakaari Management is not a passive landowner," the judge said. "The relevant duty was for WML to ensure that the health of safety of persons it permitted to be on Whakaari was not put at risk."
He said Whakaari Management had not done proper risk assessments or engaged with experts at research institute GNS Science.
"The interaction between WML and GNS was not enough to amount to taking necessary expert advice on the risk of permitting tours on Whakaari," Thomas said.
"[It] was ad-hoc, infrequent, unstructured, informal and incomplete when much more was required."
Judge Thomas said a prior eruption in 2016, which happened at night when no tourists or workers were on the island, should have been a wake-up call.
"What should have been obvious to any Whakaari stakeholder was that any risk assessment and risk management processes in place had failed," he said.
"Whatever it thought was in place prior, it needed to stop and re-evaluate."
He said WML had not done a proper risk assessment, and therefore "failed to install facilities that would sufficiently mitigate the risk, failed to ensure that workers and tourists were supplied with appropriate personal protective equipment, and failed to ensure there was an adequate means of evacuation".
"This was a major failure and amounts to a breach of its duty under section 37 [of the Health and Safety at Work Act 2015]," he said.
The judge dismissed the second charge, which alleged that Whakaari Management under section 36 had a responsibility to ensure the health and safety of workers and tourists.
He said that clause only applied if the workers were "influenced or directed" by the company.
"In a fundamental sense, WML did not influence or direct the tour operators' activities while carrying out [their] work."
WorkSafe later clarified that it had filed the section 36 charge as an alternative - only seeking a conviction for one charge or the other, not both.
Since the charge under section 37 was successful, Judge Evangelos Thomas dismissed the charge under section 36.
Judge Evangelos Thomas thanked survivors of the eruption who had given evidence throughout the trial.
"Each was remarkable, each showed great strength, poise and dignity," he said. "Their stories were confronting, poignant, and left a deep and enduring impression. I thank them for their strength and courage."
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.
Victim's mother says conviction 'means we can move forward'
The mother of a tour guide who died during the Whakaari/White Island eruption said she was grateful the company managing the island had been convicted.
Avey Woods, whose son Hayden Marshall-Inman was one of the 22 people killed, said it had been an emotional day.
"It was a good reaction, very emotional, but as far as I'm concerned because I'm talking to you as a mum that something at last is happening, we feel like we've had an answer today which means we can move forward."
Woods hoped the verdict would ensure a disaster like the Whakaari eruption did not happen again.