The police were working with the National Emergency Management Agency to coordinate a search and rescue operation following the eruption, reports The New Zealand Herald newspaper.
"While it was initially believed there were approximately 100 people on or near the island at the time of the eruption, we now believe there were fewer than 50," the police said in a statement.
"Some of those people have been transported to shore, however a number believed to be on the island are currently unaccounted for."
Families of those affected were gathering at Whakatane wharf where people covered in ash were also being treated.
"Of those transported to the wharf, at least one has been critically injured," the police added.
Earlier, Prime Minister said about 100 people had been believed to on or near the island.
"All our thoughts are with those affected at this stage," she added.
Meanwhile, GeoNet, the country's geological hazard information centre, said that the "short-lived" eruption generated an ash plume of over 12,000ft metres above the vent, adding the ash fall appeared to be confined to the island.
"We have seen a steady decline in activity since the eruption," it added.
The agency put the Volcanic Alert Level to Level 3, with the Aviation Colour Code remained at Orange.
( With inputs from IANS )