GNS Science experts have given a 40 to 60 per cent chance of another eruption outside White Island's vent area in the next 24 hours.

That "medium" likelihood was dramatically different to the 0.1 to 0.2 per cent probability of an eruption set back in late October, when the island's volcanic alert level had been set at 1.

GNS Science duty volcanologist Craig Miller said volcanic tremors had increased since this morning.

"Our monitoring equipment continues to function and is providing us with continuous data on the volcano's activity," Miller said.

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"The latest data shows ground shaking is increasing. It's important to remember Whakaari/White Island is New Zealand's most active volcano, and there remains significant uncertainty about any future activity."

So how did scientists work out probabilities?

"After there's been an earthquake, there is a very well-defined physical law of aftershock decay that you can use to make probability calculations," GNS Science volcanologist Brad Scott said.

"In a volcano situation, that doesn't exist."

After an eruption like Monday's, GNS scientists look at questions such as the possibility of further eruptions in the week, month or year to come, and which specific areas are most vulnerable.

They drew on a methodology called expert elicitation: essentially the consensus of opinions given in response to carefully-designed questions.

Scott said those opinions were formed not only by scientific expertise, but what the latest monitoring data showed.

"What results from that is an indication of risk to life at returning to the volcano," he said.

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"Firstly, we do a calculation around the potential safety of our staff and that informs our decision over whether they should or shouldn't be in the field.

"Then we look at different scenarios. In the simplest sense, you can take small, medium and large eruptions, and the team is asked what do we consider is the likeliest event.

"The probability should all point to one of those."

Yesterday, Scott said scientists had put "all of the eggs in the same cart, and saying there's a 50-50 chance of a similar sized or smaller eruption, or nothing".

"But there's an enormous amount of uncertainty around the process."

Police said this morning that serious hazards remained for those involved in the retrieval operation.

A drone had been sent over White Island to test for toxic gas this morning, and police would be speaking with GNS scientists later today before deciding whether it was safe to return.

There was also an "extremely low" likelihood of any potential ash affecting the mainland, but people may smell gas, depending on the prevailing wind direction.