Matthew Hollander, 13, and Berend Hollander, 16, died in hospital in NZ from injuries sustained after an active volcano on White Island erupted on Monday, their school said.

Matthew was in Year 8 and Berend was in Year 10 at Knox Grammar School on Sydney's north shore.

The boys were part of a group of 24 Australians touring the island off the east coast of NZ North Island.

Their parents Martin and Barbara Hollander are still unaccounted for.

Knox Grammar headmaster Scott James said the death of the boys was a "devastating loss for our community.

"Matthew was a vibrant member of the class of 2023," he wrote in a letter to school parents on Thursday.

"He was involved in cadets and representing the school in basketball, squash and debating. He was elected as a Mentor Representative in 2018/19. Matthew had a close circle of friends and was popular amongst his peers.

"He was always enthusiastic about life."

Berend, known as Ben, was passionate about AFL, cadets and most of all baseball.

"Ben's engaging smile and quirky sense of humour made him a good mate to his close group of friends and a welcome member to every classroom," the school said.

"He had a great love for the outdoors and camp. Ben was a compassionate and enthusiastic student ... (his) engaging smile and quirky sense of humour made him a good mate to his close group of friends and a welcome member to every classroom."

Knox students have been offered counselling to deal with the tragedy.

Australian Associated Press