The pair is cruising on board the Royal Caribbean ship, Ovation of the Seas, which left Sydney on December 4 and is currently docked at Tauranga, on the main island.

Mr Ostini and Mr Cardwell were not among those who had signed up for a day trip to the White Island when disaster struck on Monday.

White Island, also known as Whakaari, erupted off New Zealand's North Island on Monday afternoon, with 24 Australians caught up in the blast.

Five people are confirmed dead after the eruption.

Mr Ostini said he and Mr Cardwell are uninjured following the eruption because they were on the main island.

But the ship remains docked Tauranga following the explosion and the pair is unsure when the boat will be allowed to leave, given there's a number of passengers and crew members still unaccounted for.

"We're ok. The boat was to leave last night at six but we're still here," Mr Ostini said on Tuesday morning.

The pair was on the White Island beach when the volcano erupted.

"The captain is giving us an update at 6.30pm (on Tuesday night, New Zealand time) but we don't know when we're leaving or what happens next with the rest of the cruise," he added.

Mr Ostini said there's been regular announcements from the Ovation of the Seas captain but information in those announcements has been vague.

"There's 5000 people on board. They don't want to get everyone worked up," Ostini added.

The extent of the injuries suffered by the 13 Australians hospitalised is unknown.

"This is a very, very hard day for a lot of Australian families whose loved ones have been caught up in this terrible, terrible tragedy," Prime Minister Scott Morrison told reporters in Sydney on Tuesday morning.

"I fear there is worse news to come over the course of perhaps today or over the next few days," Mr Morrison said.

"This is a terrible tragedy, a time of great innocence and joy interrupted by the horror of that eruption."

New Zealand authorities have done a reconnaissance mission and determined that nobody on the island could still be alive.