Police have named the man who died in hospital overnight following an explosion at a Hibiscus Coast secondary school.

Richard Louis Nel, 59, was the caretaker at Orewa College. Mr Nel suffered burns to 90 per cent of his body in the blast at the school and died in Middlemore Hospital.

Another man who received serious head injuries in the accident remains in a critical but stable condition in Auckland Hospital.

Mr Nel and the other injured man - understood to be a contractor from Tubman heating Services - were flown to hospital after scorching liquid and debris shot out from the boiler-room roof they were working on.

The school will be closed today as investigators try to establish the cause of the explosion.

The force of the 2.45pm blast shook classrooms throughout the school.

Staff and pupils described hearing a loud "cracking" or "thud-like" noise that some initially thought was construction for the new arts and events centre. Others were more alarmed.

Year 9 student Jack Attwood, 13, was sitting in class when the heaters in the room began to "rumble".

"[It was] like someone was kicking them and then the pipes burst and clear oil started spilling out of the heaters and we heard a loud explosion.

"We all ran into the corridors. Everybody was freaking out and all the girls started screaming."

Year 10 student Natasha Glover, 15, had heard that a Year 12 student was "blown against the wall" by the force of the blast.

Others were without their schoolbags later in the afternoon because some classrooms were out of bounds until ruled safe. Choir and sports team practices were cancelled.

When the alarm rang, some of the students thought it was a trial run, as there had been one in the morning and one on Tuesday.

But when the college's 1600-odd students filed out to its two grassy evacuation points, they were told that the Westpac rescue helicopter had just flown the Mr Nel and the other man to hospital with life-threatening injuries.

Another contractor was treated for shock but was uninjured. No students were injured in the blast.

It is not known whether the men were propelled off the roof in the explosion, which caused school heaters to simultaneously overheat and spit out hot oil.

Orewa College principal Kate Shevland said workers were called in on Tuesday to fix the faulty boiler, but she did not know what caused the explosion.

"It sounded like something very heavy hitting the ground ... but when I went to look, there was a stream of smoke coming out of the boiler room."

Ms Shevland closed the school for today, and yesterday the electronic sign out the front of the college was broadcasting that fact. It read:

"Orewa College is closed Thurs 25 June. Due to serious boiler/plumbing problems. Check the school website for updates on www.orewa.school.nz."

TVNZ reported that students would be individually contacted and offered counselling on their return to class.

One student contacted the Herald, saying reports that the college had previously advised students and parents of boiler problems were inaccurate.

"This is not true. Being a student at Orewa College, I was not notified of this," she said in an email.

"Teachers were aware the heaters were leaking throughout the day but did not know how serious it was."

Police said the explosion was a "workplace accident" and have handed the inquiry over to Occupational Safety and Health.

The Department of Labour is investigating.