Scientists have better idea why Mt Tongariro erupted

Scientists now have a better picture of what caused last week's Mt Tongariro eruption.

Volcanologist Steve Sherburn of GNS Science said there was continuous monitoring of the volcano since the eruption and scientists were positive they could predict what could happen from week to week.

He said scientists had been able to analyse ash and gas samples and had also flown over the mountain to look at what happened.

Mr Sherburn said it had been established that the rocks thrown from the volcano last Monday were not from fresh magma, and were probably lava flows from when Mt Tongariro last erupted in the 1890s.

He said scientists knew the eruption was not just a hydro-thermal eruption as first thought.

"It wasn't just a shallow explosion, there was definitely magma gas released and that's why we're saying it was a phreatic eruption, driven by volcanic gas pressure," he said.

"Since the eruption, we've mostly seen steam and very minor amounts of ash come from the mountain."

He said it was still difficult to say what would happen with the mountain in the future.

"We can only look at things on a week-by-week basis," he said.

"At this stage, we can probably say that if anything was to happen this week it would be small-scale eruptions, like last week. Other than that we cannot predict what would happen."

He said White Island remained an active area but the volcano had not been active for the past 10 years. He said scientists were still trying to work out if the recent activity was the start of an "episode of activity".

"Times between active periods could be weeks or months but it is really difficult for us to see at the moment where this is heading."