New Zealand's workplace regulator has filed charges against 13 parties following an investigation into a volcanic eruption on White Island in 2019 that killed 22 people.

A surprise eruption on the White Island, also known by its Maori name of Whakaari, on December 9, 2019, killed 22 people and injured dozens.

The majority of the casualties were tourists from countries such as Australia, the United States and Malaysia who were on a cruise ship vacation, traveling around New Zealand. There were 47 people on the island when the volcano erupted.

WorkSafe, New Zealand's primary regulator for workplace related incidents, said at a news conference its investigations found 13 parties had not met their health and safety obligations in taking the tourists to the White island.

"This was an unexpected event, but that does not mean it was unforeseeable and there is a duty on operators to protect those in their care,” WorkSafe Chief Executive Phil Parkes said.

WorkSafe charged 10 organizations under the Health and Safety at Work Act with each charge carrying a maximum fine of NZ $1.5 million ($1.06 million).

Three individuals were charged as directors or individuals who were required to exercise due diligence to ensure the company meets its health and safety obligations. These charges each carry a maximum fine of $300,000.

WorkSafe did not name those charged because they may seek suppression orders in their first appearance in court December 15.

The agency did not investigate the rescue and recovery following the eruption, because that is the subject of a coronial inquest that is under way.

At the time of the eruption, questions were raised about why people were allowed on the island, a popular destination for day trips, given there was reportedly a heightened risk of an eruption.