Volcanic Air and six other private helicopter company staff members flew their aircraft to Whakaari/White Island on the day of the eruption and rescued survivors.
Some of those onboard died en route to hospital but many survived.
Victims' relative 'pleased' at news of prosecutions
Meanwhile a woman who lost her twin brother in the Whakaari/White Island eruption says she is "quite pleased" at news that WorkSafe has filed charges against 13 parties involved.
The investigation gave "some relief" to Meredith Dallow, the twin sister of Gavin Dallow who lost his life in the eruption.
She said news of the charges made her "quite pleased".
"I'm not surprised that there have been charges laid and I'm actually quite pleased to be honest."
WorkSafe is holding a press conference at 3pm today to release the findings of its investigation.
Ten parties are facing charges under the Health and Safety at Work Act and are facing a maximum fine of $1.5 million, TVNZ reports.
Three people have been charged as directors or individuals who were required to exercise due diligence to ensure the company meets its health and safety obligations.
Those charges each carry a maximum fine of $300,000.
Adelaide-based lawyer Gavin Dallow, 53, and his 15-year-old step-daughter Zoe Hosking died when Whakaari/White Island, off the Bay of Plenty coastline, erupted underneath them on December 9.
Gavin's wife Lisa Dallow, 48, was critically injured, suffering burns to almost 60 per cent of her body.
Gavin was farewelled a month after the eruption, when more than 600 people attended his funeral at world-famous sporting venue, the Adelaide Oval.
Today Meredith said she was assuming who had been charged at this stage, but the maximum fine of $1.5 million was "better than nothing".
"I'm glad the WorkSafe investigation went ahead and there is an outcome but it doesn't really put closure to things."
Meredith did not believe closure would come until the court cases and coronal inquest were over, but she was thankful the WorkSafe report was released now rather than next week, around the anniversary of the eruption.
"It does give us some relief especially as we come close to the 12 month anniversary.
"It's been a pretty thorough investigation, so we're happy with the way the investigation has been done."
TVNZ reported that the charged parties will appear in the Auckland District Court on December 15.
Forty-seven people were on the island when it erupted on December 9 last year and 22 later died from their injuries.
Nineteen tourists and two tour guides from White Island Tours were among the dead after the volcanic island erupted underneath them during a sight-seeing tour.
Many more would have died if not for the heroic actions of tour guides, fellow tourists, helicopter charter pilots who flew out to the island in an unsanctioned rescue mission, and then the work of specialist medical teams.
Tourists have not been back to the island since.
WorkSafe confirmed the day after the tragedy that it had launched its own investigation.
In a statement at the time, the department said: "WorkSafe New Zealand has opened a health and safety investigation into the harm and loss of life caused by the eruption.
"As the workplace health and safety regulator and administrator of Adventure Activities, Regulations, WorkSafe will be investigating and considering all of the relevant work health and safety issues surrounding this tragic event."
Under law, WorkSafe can prosecute for breaches of health and safety regulations. Penalties and sanctions range from $50,000 to $3m and jail sentences of up to five years.
WorkSafe had general oversight of tourism operations on the volcanic island off the Bay of Plenty coastline.
Under regulations introduced in 2016, it required companies such as White Island Tours and the myriad of helicopter companies that offered tours to the island to undergo safety audits.
But it could not dictate access to the island as it was owned privately.
Prime Minister Jacinda Ardern also said WorkSafe would also be reviewing its own regulatory framework in light of the disaster.
"Did the operators fulfil the expectations around health and safety audits and so on? The question may well be was that regime itself adequate and did WorkSafe do the job," Ardern said.
In June it was revealed that WorkSafe's investigation team included 27 staff members.
Six months after the eruption, NZME also revealed that a former police sergeant and Whakatāne district councillor said he had raised concerns about tourists on White Island before the deadly eruption.
Emails provided to NZME and an interview with Russell Orr – who served 21 years in the police before being a councillor on the Whakatāne District Council between 2004-2019 – showed he was worried as early as 2013.
An email from then showed he requested that council staff make sure risks were being "conveyed fully" to tourists, fishermen and other visitors to the island.
Orr requested in one email: "If the risk is 'unacceptable' that business and economic issues do not sway us from making the obvious decisions needed to protect lives."
On January 25, 2013, a private meeting including police, Civil Defence, GNS Science and Whakatāne District Council staff addressed safety issues.
Orr told NZME that as far as he was aware no action had been taken after the meeting.
"Informally I was told [by council staff] council had no jurisdiction, that it wasn't in our area."
The revealed on October 17 that numerous lawyers in Australia and America had been hired by White Island survivors and family and friends of some of the 21 people who died.
Sydney-based lawyer Rita Yousef confirmed she had two confirmed clients, and had also been approached by others affected by the tragedy – including some still receiving hospital care – who want to sue Royal Caribbean Group.
Most of those killed or injured in the eruption were on a sight-seeing voyage around New Zealand on the company's Ovation of the Seas vessel. Royal Caribbean Group is the world's second-biggest cruise operator.
The crux of the case will be on the level of warnings passengers were given by the vessel's crew before signing up for the tour.
"As far as I know there were no warnings about potential eruptions," Yousef said.
"The only warning I am aware of that they were provided with was that the terrain might be a bit difficult. People with mobility issues were advised against it, and they were told to wear enclosed shoes.
"Apart from that there were no warnings about the risk of eruption and injury [from the cruise company]."
The claims were restricted to Royal Caribbean Group. That was because it was understood those who went on the fateful trip had bought their tickets direct from the cruise line, and not from the Whakatāne-based White Island Tours.
What happened to those who had ventured on to White Island was extremely traumatising, she said.
"Every time my mind turns to thinking about what those people would have suffered, it is just absolutely horrendous," Yousef said.
"Basically being burned alive, people witnessing others melting before their eyes. And not to mention the respiratory problems they would have experienced just by inhaling all the gases and vapours.
"People think of burns as affecting the skin and causing scarring, but also the internal injuries are horrific; injuries that a lot of people can never truly overcome."
During her unsuccessful election campaign, National Party leader Judith Collins had called for a Royal Commission of Inquiry into the White Island tragedy.
"I think it needs to have that level of oversight, and I think that people would probably be very grateful if the prime minister announced that," Collins said.
"If I am fortunate enough to be the next prime minister, I will certainly announce that – if it is not done before then."
