First White Island rumbles, now Mt Ruapehu is making volcanologists sit up

The temperature of the crater lake at the top of Mt Ruapehu has risen in temperature. Photo / Getty Images

First it was White Island's crater lake that was making scientists sit up - and now it is the one atop Mt Ruapehu.

But the timing of White Island's latest bout of unrest, which culminated in an overnight eruption this week, and a volcanic earthquake swarm 241km away beneath Mt Ruapehu's crater lake should be seen only as a coincidence, a volcanologist says.

This afternoon, GNS Science reported that the mountain's crater lake had risen from 25C in mid-April to 40C.

The lake temperature had been rising since late 2015, at a similar rate to that observed in March 2011, April 2014 and February 2015.

And over the past few days, scientists had been recording an earthquake swarm beneath the lake - something that has been an uncommon occurrence there over recent years.

"Seismic activity at Mt Ruapehu is usually dominated by volcanic tremor," GNS volcanologist Brad Scott said.

Scientists had however not noted any changes in other monitored parameters like volcanic gas, lake chemistry or lake overflow.

"Currently we are uncertain of the implications of the recent observations."

The developments weren't sufficient enough to upgrade Mt Ruapehu's Volcanic Alert Level, which remained at 1.

They come in the same week that an eruption at White Island late on Wednesday night threw material across the offshore volcano's crater.

But Mr Scott said this event wasn't related.

The two occurrences could be compared to a pair of houses that both fed into the same water main, with one having problems with its sink and the other with its bath.

"We have examples of volcanoes behaving together and equally we have examples of volcanoes not behaving together," Mr Scott said.

"In the 1940s, for example, Mt Ngauruhoe was having lava flows and Mt Ruapehu was having its first eruption in 50 years, and then come 1995, Mt Ruapehu was erupting again and Mt Ngauruhoe was dead as.

"Of all the people who have looked at the relationships, nobody has been able to demonstrate a repeatable one."