The volcano, also known as Whakaari, erupted in December 2019, killing 22 people - including 14 Australians - and seriously injuring many of the 47 people visiting.

A year after the disaster, New Zealand watchdog Worksafe brought charges against 13 individuals and organisations for safety failings in the lead-up to the eruption.

A dozen of those charged - including the island's owners, the National Emergency Management Agency and a suite of touism operators - are contesting the charges ahead of an expected trial in mid 2023.

The thirteenth, Inflite Charters, changed its plea to guilty, and was sentenced to pay a fine of $NZ227,500 ($A211,000) and prosecution costs totalling $NZ40,000 ($A37,000).

Inflite did not have any tourists on the island at the time of the eruption, but was regularly engaged in tourism to the island, found 50km offshore to the north of Whakatane.

"It did not carry out adequate risk assessments on Whakaari itself, it did not have safety management systems or safe operating procedures itself for tours to Whakaari," Mr Thomas said, reported by TVNZ.

"There were gaps in the necessary measures that should have been in place on the island. There was a lack of emergency appropriate planning.

"The only emergency shelter for example on Whakaari was a metal container unit used to store some emergency supplies, there was otherwise inadequate shelter in the event of an eruption on the island.

"Mother Nature doesn't play by our rule book, and we have to be prepared for consequences that we may not have been anticipating but which we know are possible."

WorkSafe welcomed the sentence.

"It's encouraging to see Inflite Charters Limited plead guilty at this reasonably early stage in the court process and take accountability for not doing enough to keep people safe," chief executive Phil Parkes said.

Australian Associated Press