"Visual observations last week and Monday show lava has been extruded into the vents created by the December 9 eruption," a statement read.
"In previous bulletins, we proposed that magma was at a very shallow level beneath the floor of the crater, and we now have visual confirmation that this magma has risen to the surface."
When magma reaches the surface, it is called lava.
Apart from minor ash emissions on December 23 and 26, no significantly sustained or strong eruptive activity had been observed since the December 9 eruption.
White Island Aerial view after the volcanic eruption on December 9. Photo / George Novak
The eruption killed 20 people – including fellow tour guide Hayden Marshall-Inman and teenage Australian tourist Winona Langford, whose bodies have not yet been located.
"Airborne measurements of gas on Monday show they are still at the high levels expected after an eruption, and the vent temperature remains very hot (> 400 C).
"Considering these new observations, we have recalculated the expert judgement of eruption likelihood.
"It remains very unlikely (1 per cent chance) that there will be another eruption in any 24-hour period during the next four weeks."
Although very unlikely, an explosive eruption from the active vent area remains possible and could occur with no precursory activity the statement continued.
"This style of eruption could be driven by the collapse of unstable material behind the vents, or by a marked decrease in gas emissions allowing water from the reforming crater lake or geothermal system to enter the active hot gas vents.
"Should any explosive activity produce an ash cloud, the likelihood of ash affecting the mainland remains low," the statement concluded.
All monitoring equipment on the island is operating and the camera feeds have been re-established on the GeoNet web pages.
