Questions are being asked over why tourists were allowed at White Island when its "alert level" had been elevated over recent weeks.

Under the GeoNet-managed NZ Volcanic Alert Level system, ranging from zero to five, the volcano had recently been rated level 2, indicating "moderate to heightened volcanic unrest".

That had been raised in response to increasing amounts of sulphur dioxide gas, along with volcanic tremors – both which can signal rising magma deep in the volcano.

It is ultimately up to operators to decide whether to take visitors to the privately-owned island, with access controlled through permits.

In 2017, the Government and the Bay of Plenty Civil Defence Emergency Management Group signed a memorandum of understanding setting out responsibilities around readiness and response to an eruption.

But the volcano didn't come under the authority of any city or district council, which would usually have had these responsibilities under the Civil Defence Emergency Management Act 2002.

At the time, the group's chair, former Tauranga mayor Greg Brownless, said staff would be liaising with tour operators "to ensure the risks to visitors on the island are managed appropriately", along with GNS Science, which regularly communicated to the public about the island, and with police, to ensure any search and rescue plans for the island are prepared and current.

Brownless said then he was confident this work had "already been carried out as required".

It's believed many tourists on the island on Monday were from cruise ship Ovation of the Seas.

One Whakatane company, White Island Tours, stated on its website that it operated through varying alert levels, but added that "passengers should be aware that there is always a risk of eruptive activity regardless of the alert level".

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"White Island Tours follows a comprehensive safety plan which determines our activities on the island at the various levels," the company said.

One of the five people killed during the eruption is believed to have been a guide from the tour company. Eight people are missing, police confirmed at a press conference this morning.

White Island Tours chairman Paul Quinn said the company was deeply saddened following the significant eruption.

"Devastation is an understatement. This is a terrible tragedy and our thoughts and prayers are with everyone who has been impacted."

He said the company was currently assisting police and Civil Defence with the official emergency response.

"We acknowledge the considerable efforts from Police and Civil Defence and will continue to do whatever is necessary throughout the rescue operation.

"Our immediate focus is on supporting our staff, manuhiri and respective whānau, who have been significantly impacted and are showing immense strength and courage."

Last year, the Whakatane business was named New Zealand's Safest Place to Work in the Small Business category of one group of workplace safety awards.

"Taking visitors into the crater of an active volcano is often perceived as high risk so we have always maintained a strong focus on ensuring our health and safety systems were of a very high standard," its general manager Patrick O'Sullivan said at the time.

At one point earlier this year, when the alert level was raised, White Island Tours said it would continue to operate, albeit with extra staff to check conditions before tour groups stepped ashore.

It advised tourists to wear appropriate footwear and provided hard hats and gas masks for safety gear.

Around 10,000 tourists visit the volcano – New Zealand's most active – each year.

Despite that, alert levels change on a relatively frequent basis – and one volcanologist said sudden unheralded eruptions from volcanoes such as White Island could be expected at any time.

While the island was generally open and exposed, a shipping container had been placed there in 2016 to act as an emergency shelter in case of an eruption.

Today, GNS Science said there remained "significant uncertainty" as to future changes but currently, there were no signs of escalation.