An eruption rocked White Island on Monday afternoon, injuring multiple people and sending a plume of smoke and ash high into the sky.
Geonet's volcanic alert bulletins warned of increased activity three weeks ago - elevating the risk to a level two of a possible five.
Despite the alert rising, eruptions can be expected at any time said volcanologist Professor Shane Cronin.
"Sudden, unheralded eruptions from volcanoes such as White Island can be expected anytime," he said in a statement on Monday evening.
"Magma is close to the surface and the heat and gases from this heat the surface and ground waters to form vigorous hydrothermal systems," Cronin said.
When ground waters are super-heated, they rapidly transform into steam, causing an eruption.
Cronin says eruptions like the one on White Island have multiple hazards.
The "violent" ejection of hot rock and ash, hurricane-like currents of wet ash and coarse particles expelled from the explosion site can cause impact trauma and burns.
The particles of ash can cause respiratory issues.
St John reports 20 people who were on the island are in need of medical assistance.
Up to 50 people were on the island at the time of the eruption and some of those are still unaccounted for.
