TORONTO -- A Canadian man is among thousands of cruise ship passengers docked in a New Zealand port after a nearby volcano erupted, killing at least six people and leaving eight others missing – some of whom may be fellow travellers.

A group of passengers from the Royal Caribbean cruise ship Ovation of the Seas were among those exploring the volcano when it suddenly erupted, officials confirmed. It’s unclear if the cruise ship passengers are among the dead.

Canadian passenger Sylvain Plasse was climbing a mountain Monday when he spotted massive white plumes about 90 kilometres away. He initially assumed that the smoke was pollution from smokestacks, but he later learned it was from a volcano.

“I thought it was really cool and exciting to see that. It was only when I came back onto the ship that I learned guests and crew from this ship were on a tour in the crater of the volcano,” Plasse, an actor who lives in Toronto, told CTV News.

Since then, the ship has remained docked in Tauranga, a coastal port city. Plasse said the captain has provided regular updates but hasn’t confirmed whether any passengers were injured or killed.

“We don’t know if they’ve passed away. We haven’t been given any details yet,” Plasse said.

Plasse said he’s particularly concerned about a young couple on their honeymoon he met earlier on the trip who told him that they planned to go on an excursion at every port stop.

“We didn’t see them last night,” he said. “So I’m hoping they just did not show up. It’s pretty scary.”

The eruption occurred on the fifth day of the 12-day cruise.

TREMORS REPORTED, BUT VOLCANO WASN’T CLOSED

The White Island volcano is a popular tourist destination in the region. But new questions are being raised about whether travellers should have been allowed to walk on the volcano’s crater in the first place.

Last month, the alert level for the volcano was raised from 1 to 2 on a scale of five, and volcanic tremors had increased from weak to moderate strength in recent weeks.

Paul Ashwell, as assistant professor in earth science at the University of Toronto Mississauga, said this volcano acts in a “very cyclical manner” and that increases in volcanic activity “happen very regularly.”

“More often than not, no eruption occurs,” he told CTV News Channel. “The actual activity that was increasing is no more than what it has done before.”

The unpredictable nature of the volcano, which is actually just the tip of a massive underwater volcano, means that it is typically monitored closely and closed off to tourists when the threat is considered too high.

Regardless, Ashwell said officials may now have to consider whether or not the risk is worth keeping the volcano open to the public.

“It is something that will be looked at in the future to see whether tourism to White Island can continue at all,” he said.

Dozens of survivors, some of them critically wounded, were airlifted off White Island by a helicopter.

New Zealand Prime Minister Jacinda Ardern said that aircraft have flown over the island, but “no signs of life have been seen at any point.”

Among those missing and injured are travellers from Australia, the U.S., China, Britain and Malaysia. So far, no Canadians are reported missing.

With files from CTV's John Vennavally-Rao and The Associated Press