Prime Minister Jacinda Ardern says "around 100 people were around or on" White Island at the time of the eruption this afternoon, and not all of them are accounted for.
Ardern is speaking at her weekly press conference.
"All our thoughts are with those affected at this stage," Ardern said.
Ardern apologised for her late start to the regular weekly meeting, saying she wanted the most up to date information on the eruption.
Police were alerted of the eruption at 2.17pm, she said.
There will be a joint press conference at 5.30pm with Civil Defence and police.
As many as 20 people have been reported injured following an eruption at the volcanic island in the Bay of Plenty this afternoon.
Ardern is expected to discuss the situation when she faces reporters at her regular post-Cabinet-meeting press conference in the Beehive.
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She is also expected give an update on the response to wild weather that has wreaked havoc in the South Island.
Downpours in the South Island over the weekend saw about 1000 tourists trapped at Franz Josef, power cut to townships and sections of major highways blocked off.
Ardern will also likely face questions about allegations by former Auditor-General Martin Matthews that he was forced out of his job by politicians in 2017, when she addresses media at her weekly press conference in the Beehive.
Matthews on Monday held a press conference to petition Parliament to reopen his employment case, claiming he was forced out of his job in the fallout over how a serial con-woman was able to fleece $750,000 from a government department.
Matthews says he feels like his "scalp" was offered up by politicians who needed to get rid of him to please voters before the last election, after it was revealed Joanne Harrison had defrauded the Ministry of Transport while Matthews was its head.