Hydrothermal activity is increasing at White Island which could be the start of an eruptive process, says GNS volcanologist Brad Scott.

The volcanologist visited the island volcano off the Bay of Plenty coast on Monday following reports of increased activity from tour operators, and a rise in activity showing on the island seismometer.

Increased hydrothermal activity in the White Island crater. Photo: Brad Scott, GNS Science.

'Unrest has increased and this could go one of two ways,” says Brad today.

'It could fizzle out and go away, or it could lead into the next eruption. It's a 50:50 call either way at the moment.”

The activity is the strongest Brad has seen since the late 90s, which led to a large eruption in 2000, says Brad.

"The hydrothermal activity is some of the most vigorous I have seen at White Island for many years. This type of activity usually leads to stronger volcanic activity and is a significant concern.

"Increases in hydrothermal activity often lead to explosive eruptions and that can be quite dangerous for visitors."

The eruption on August 5, 2012 threw a few rocks out which could have affected people if they had been there, says Brad.

White Island is 49km north of Whakatane, and about 321 metres above sea level. Its full height is about 760 metres above the sea floor. The visible part of the submarine volcano is 2.4 km by 2km. It's footprint on the seafloor is 14km by 16km.

It is New Zealand's most active volcano, but its eruptions do not affect the mainland, says Brad.

Ash from the volcano has in the past been mistaken for pollen and goes unnoticed by most people.

The likelihood of the mainland being affected by an eruption through a tsunami would be minimal.

A tsunami study done by the University of Waikato found the wave that reached the Bay of Plenty beaches would be about knee high.

'They looked at the tsunami more so from the land slide risk,” says Brad.

'If there was an eruption induced land slide it clearly demonstrated there was no significant risk to the coast.”

Brad says what he viewed yesterday is increased hydrothermal activity in the small 'hot lake'. Large amounts of sediment have surfaced and vivid white steam and gas was flashing from the base.

This sort of activity has been increasing since late 2012 and is now semi-continuous, says Scott.

The lava dome first noticed in late November has not changed since earlier this month.