Whakaari/White Island is continuously passively erupting a small amount of volcanic ash, prompting Earth Sciences NZ to raise the volcanic alert level.

It said there are no threats to the mainland beyond potentially noticeable sulphur smell and light ash fall.

The aviation colour code has been raised to orange.

The change in activity was confirmed during an observation flight on Tuesday afternoon.

"A blanket of volcanic ash could be seen around the active vent in the Main Crater, and a small amount was being carried upwards in a column of steam and gas.The discharge was weak, with no evidence that any explosive eruption had occurred," Earth Sciences NZ said.

This activity is typical of eruption cycles seen at Whakaari/White Island and may continue for weeks, it said.

It said volcanic activity could escalate with little or no warning, however any significant impact to the mainland remains very unlikely.

"The level of volcanic activity would have to escalate significantly for this likelihood to increase."

More explosive and sudden events can throw rocks and produce gas surges which can threaten life on the main crater floor and immediately offshore on the South-East of the island, it said.

It could also cause ash fallout downwind in the immediate vicinity of the island which can be an inconvenience to aircraft and boats in the area, it said.