New Zealand's central North Island looks set for a period of volcanic activity after warnings issued on three volcanoes over the last two days.

On Wednesday, scientists at the government's Institute of Geological and Nuclear Sciences (GNS Science) announced they had noticed a "spiny lava dome" that had grown in a crater on White Island, a volcanic island off the east of the North Island, after an eruption on August 5.

GNS Science raised the volcanic alert level to level two, indicating minor eruptive activity, and the aviation color code to orange, warning pilots that an eruption was underway with little or no ash emissions.

"The dome is probably 20 to 30 meters across and has spines sticking up. In more than 30 years I've never seen anything like it while visiting White Island," GNS Science volcanologist Brad Scott said in a statement.

"If you imagine a volcano is like a tube of toothpaste then a lava dome is like the congealed, dried toothpaste that has come out of the tube. It doesn't move like a lava flow, but grows from the inside as new magma comes up and pushes the already cooled magma upwards and outwards," said Scott.

Lava domes were common at some volcanoes, but had never been seen before at White Island, where magma last reached the surface in 2000 during an explosive eruption that threw up molten rocks called volcanic bombs, rather than forming a dome.

"Our concern is that lava dome growth can be accompanied by explosive eruptions and could impact people on the island. We need to see if the dome is continuing to grow. If it's not growing, then we will be less concerned," GNS Science head volcanologist Gill Jolly said in the statement.

GNS Science would continue to closely monitor White Island.

Also Wednesday, GNS Science volcanologists warned they still believed Mount Ruapehu, in the central North Island, remained at a heightened level of unrest and that an eruption was more likely than normal.

"Our analysis is still showing higher than normal temperatures beneath the crater lake. We think this reflects a partially sealed zone a few hundred meters beneath the lake, which might be causing a pressure build up behind it. That pressure would make an eruption more likely than normal," Jolly said in a statement.

"It doesn't mean that an eruption is inevitable," said Jolly.

"If the sealed zone fails suddenly an eruption could occur, probably with little or no warning. If it fails more gradually then the pressure would probably be released more slowly and the likelihood of an eruption would revert to normal."

The crater lake was quiet and its temperature had remained relatively low at 20 to 25 degrees centigrade since March.

Eruptions at Ruapehu in 1988, 2006 and 2007 were believed to have occurred as a result of sudden failure of a seal beneath the crater lake.

Small earthquakes 3 to 5 km beneath the crater lake in late October and early November had stopped, but it was unclear whether they were related to the high temperatures estimated a few hundred meters beneath the lake.

The volcanic alert level for Ruapehu remained at level one, indicating a departure from typical background surface activity, and the aviation color code at yellow, indicating signs of elevated unrest above known background levels.

On Tuesday, GNS Science warned that neighboring Mount Tongariro was still showing a substantial possibility of further eruptions after an eruption in August and two more last month.

"After the second eruption in November we now have to consider the possibility that Tongariro might have entered an eruptive episode and this unrest could continue for several months," Scott said in a statement.

"Within an episode Tongariro might quietly discharge steam most of the time, but occasionally have small eruptions with little or no warning. There was a similar episode of activity in the 1890s."

Tongariro had seen only minor seismic activity since November 21, but there had been a very noticeable gas discharge in recent days.

It was on volcanic alert level one and aviation color code yellow.

The 80,000-hectare Tongariro National Park has more than 1 million visitors a year.

Last month's eruption resulted in videos of panicked hikers fleeing from the plume of ash and steam.

Tongariro's eruption on Aug. 6 was the first since 1897.