Earlier today, it was believed around 20 people were in the area following the eruption. However, this number has since risen to around 100. Ms Ardern addressed the media earlier and confirmed the number.
She said: “Police were alerted at 2.17pm.
“At this stage, and please do keep in mind this is an evolving situation, we believe around 100 people were on or around the island at the time and some of those are at this stage unaccounted for.
“I want to share this is evolving at this stage, at this stage it does appear to be a very significant issue, particularly the scale of those affected.”
A national warning has since been issued for the eruption.
New Zealand police are advising people that the volcanic ash could pose a major health hazard to the public.
Roads have been close in and around the town of Whakatāne, which is the closest town to White Island.
Whakatāne hospital has also been turned into an emergency operation centre while events unfold.
New Zealand police said: “Effects of a volcanic eruption can be experienced many kilometres from a volcano.
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“Be aware of the potential for ashfall. Consider staying indoors. Volcanic ash could be a health hazard, especially if you suffer from breathing difficulties. When indoors, close all windows and doors to limit the entry of volcanic ash.
“If caught in volcanic ashfalls: Wear a dust mask or use a cloth handkerchief over your nose and mouth; protect your eyes by wearing goggles. Wear eyeglasses, not contact lenses as fine ash will get under the lens.”
Seven helicopters have been dispatched to the island by St John Ambulance.
Whakaari/White Island is the country’s most active cone volcano and is therefore, a very popular tourist and scientific research site.