Lives would have been saved if Department of Conservation protocols were in place at Whakaari last year, government documents suggest.
Twenty-one people died from blast injuries and burns after searing gases, rock and ash erupted from the volcano, on December 9.
All 21 were visiting with White Island Tours - a business bought by Whakatāne iwi Ngāti Awa in 2017, from founders Peter and Jenny Tait.
Whakaari / White Island's alert level was raised from 1 to 2 three weeks before the eruption but sightseeing groups continued to visit.
Emails released to NZME under the Official Information Act show volcanoes at level 2 are deemed too dangerous to allow tourist access, under DoC rules used at Tongariro National Park.
However, DoC does not control access to Whakaari, it is owned privately by the Buttle family.
In May this year, a senior DoC public safety staff member emailed the Department of Internal Affairs to say: "If you overlay our approach to Whakaari in December 2019, no one would have been permitted on the island as the volcano was at VAL [volcanic alert level] 2."
They said DoC managed hazard exposures "as unrest rises and falls" and this was difficult at Whakaari as visitors were close to the source of the eruption, "significantly increasing the residual risk even with risk management in place".
The email was in an Internal Affairs Official Information Act response to NZME.
DoC subsequently provided NZME with a copy of the volcanic hazards "decision tree" rangers and senior staff use.
Protocols prohibit recreational use of Tongariro Crossing at volcanic alert level 2.
Even at level 1, the track is closed while data and advice are sought from experts.
The track is only reopened if the likelihood of an eruption is low and DoC can tolerate the risk.
Australian Meredith Dallow, whose twin brother Gavin Dallow died in the eruption, said it was tough knowing tourists wouldn't have been on Whakaari last December if Department of Conservation protocols were in place.
"It doesn't make things any easier hearing that. It makes you a bit frustrated and angry."
Her brother, an Adelaide lawyer who was on the volcano tour with his wife Lisa Dallow and 15-year-old stepdaughter Zoe Hosking, would never have gone if he'd known the risk, Dallow said.
"We did wonder why they were allowed on it. To me, they should never have been there ... and I don't think it should ever be re-opened."
Only Lisa Dallow survived the eruption, suffering burns to 60 per cent of her body.
New Zealand's most active volcano, Whakaari, has been owned by the Buttle family since 1936.
In the past 30 years, a handful of tourism businesses have paid the Buttles for exclusive landing rights at the crater.
These businesses have decided, independently, when it is or isn't safe to guide tourists around the submarine volcano.
Tourists have not been back to White Island since the eruption on December 9.
WorkSafe has been investigating whether the operators breached health and safety rules and the police are investigating the deaths on behalf of the Coroner.
Past reporting by NZME publications shows that under the Taits' management, White Island Tours took tourists to Whakaari at raised alert levels as far back as 1999.
When GNS put Whakaari up to alert level 2 in January 1999, plumes of steam and ash were shooting up to 600m from a new vent.
Jenny Tait told the NZ Herald trips were still on though.
For every nervous tourist, there were bookings from others curious to view the fresh activity, "which makes White Island so neat", she said in the January 9 paper.
By 2006, White Island Tours boats were taking about 15,000 tourists to the island each year.
In the January 26, 2013, edition of the Rotorua Daily Post, White Island Tours marketing manager Patrick O'Sullivan said White Island Tours had visited the island within hours of an eruption in August 2012.
"Even when there was a large scale eruption in 2000 we were out there in a couple of days," he said.
That eruption in 2000 covered the crater floor in scoria, displaced the crater lake and formed a new explosion crater 150m wide.
Then in August 2013, the Bay of Plenty Times reported on an eruption of mud and rocks that sent a plume of steam 4km skyward, as a White Island Tours boat heading to the volcano watched from 2.5km offshore.
Passengers and crew watched the 10-minute eruption and were taken for a scenic sea tour instead.
The island was put at level 2.
O'Sullivan said it would be business as usual for White Island Tours the following day, unless GeoNet indicated otherwise, August 21's paper reported.
Yet GNS staff were not visiting until they had more data.
The August 21, 2013, NZ Herald quoted GNS volcanologist Craig Miller stating scientists had found magma "very near" to the surface.
"We're not expecting it to stop in the next day or two."
He said GNS had told tour operators they could have no time to evacuate in an eruption.
"It could be the one day that it goes suddenly wrong and you have very little time to prepare for it or do anything about it once you're out there.''
The differing safety approaches between operators and scientists were again highlighted in the Herald on Sunday in 2014.
In an article about Whakaari, in the September 7 edition, GNS volcanologist Brad Scott said GNS Science staff had "terse discussions" with tour operators who had taken visitors to island in conditions that made volcanologists anxious.
He said anything between a level 2 and 3 would put him off.
"We don't visit but they still go."
Earlier this year, NZME reported academics had referenced seven life-threatening "near misses" at Whakaari in the years leading up to the eruption and published mounting evidence of "severe" risks.
When NZME approached the Buttles to discuss risks at past alert levels at the island, and if DoC's volcano risk management system could be used at the island in future, they declined to comment.
The Taits were approached about White Island Tours' visits at various alert levels, under their ownership, but did not respond.
However, a White Island Tours' procedures document supplied by police under the Official Information Act, stated that when volcanic activity increased, this information was put on the front page of the business' website, passengers were given updated safety terms to read, they were reminded of increased risk over a microphone before disembarking and were given the option to stay aboard.
When NZME approached White Island Tours and its current owners Ngāti Awa, they also declined to comment, due to ongoing investigations.
In a paper published in July this year, Auckland University proposed a new eruption alert system researchers said could have given 16 hours' warning of December's eruption.
The system used new technology to process algorithms and data patterns, signalling almost instantly when a particular pattern matched the build-up to a previous eruption.
With data from the past 10 years at Whakaari, the new system predicted four out of five eruptions - missing only one in 2016, when the data pattern was different from the other four.
GNS Science's volcanology team leader Nico Fournier said the new technology could be a "potentially useful addition" to current systems, but cautioned it still had limitations.
Whakaari has been at alert level 1 since June 16, 2020.
Between 1995 and 2014, alert level 2 represented a minor eruption on active cone volcanoes in New Zealand such as Whakaari, but new definitions were brought in in July 2014. Now level 3 represents a minor eruption, and level 2 is moderate to heightened unrest.
The 19-year-old was found in a remote area in the hills of Upper Hutt.