White Island's famous crater lake is back - but the giant volcano is still showing no signs of settling down.

The lake, noted for its hot temperatures, brilliant green colouring and acid levels 50 times more potent than battery acid, had dried up amid heightened activity that elevated the Bay of Plenty island volcano's alert level last year.

When GNS Science volcanologist Brad Scott visited the island on Tuesday, much of the crater was again drowned by a lake more than 150m long. He suspected run-off into the crater from winter rains had helped restore the lake.

"The heat flow from the volcano is maybe also not as high as it was last year," Mr Scott said. "The place has possibly cooled down slightly, which maybe means it's not evaporating as quickly, but there is still a lot of hydrothermal activity occurring from the active vent."

Advertisement

A lava dome, which appeared last November and is believed to be plugging magma below the surface of the volcano, was still visible this week. And high maximum temperatures - measuring nearly 170C within volcanic fumaroles and 49C around them - also showed the volcano's unsettled behaviour had not changed.

Last year, scientists monitoring the island were surprised by a series of twists, with activity swinging from relatively settled states to sudden ash eruptions.

The 2km-wide, 321m-high circular rock - the visible tip of a submarine volcano rising 1.6km from the ocean floor - sprang back into life last July and August, ending more than a decade of peace and raising aviation alert levels.

Scientists have also been able to better watch the volcano with newly installed low-light camera technology, which takes more images and can view activity at night.

Meanwhile, activity at Mt Ruapehu, where the ski season is about to open, and at Mt Tongariro, where the popular Tongariro Alpine Crossing recently fully reopened, has returned to normal levels.

Water and gas samples taken from Mt Ruapehu's crater lake have again been producing routine results, after unusually unchanged levels left scientists worried that rising pressure was being trapped below.

GNS volcanologist Dr Gill Jolly said new vents formed at Tongariro's Te Maari Crater by a surprise eruption last year were also typically active, indicating the mountain was "breathing".