A popular Auckland teacher collapsed in the middle of a gym session and died on Friday night.
Others at the gym attempted to revive Waiuku College teacher John Robinson with CPR.
Waiuku College principal Tom Vanderlaan said the tragedy had "come as a real bolt out of the blue".
"He was a member of the local fitness centre. He just collapsed very quickly. They tried to revive him but were unable to do so.
"He was very fit, active person, it's shocking for someone like that to pass away," Vanderlaan told the Herald.
Robinson was the head of department for technology and was known as being very practical and generous. His speciality was furniture and construction.
He had come to Waiuku College more than 10 years ago after living in Invercargill, but he and his wife were originally from England.
Vanderlaan said he was a volunteer firefighter and regularly organised projects with the students such as building a sleep-out house and duck shooter maimai.
"He was a real community person who gave to others and was always willing to be helpful for community causes. He was a very supportive, reliable staff member who everyone felt they could trust and confide in."
Robinson is survived by his wife Sharon and three children, the youngest of whom goes to Waiuku College. The deputy principal had visited the family who are reeling from shock, Vanderlaan said.
"Our thoughts are very much with his wife Sharon and their three children. It must be just awful what they are going through and we'll be here to support them."
Colleagues, students and friends posted to Facebook for the "amazing man with a friendly ear and a kind word".
One woman said it was devastating news.
"He has really made his mark on this world by the positive impact he has made on so many people's lives."
Another parent said he was one of the best teachers who could get "great work out of almost everyone".
Vanderlaan was meeting with senior members of staff this afternoon to decide how they would conduct school this week. He said they would have a large presence at Robinson's funeral to support him and his family.