Dozens of college students had a narrow escape when fire ripped through a wharenui at Opotiki College where exits were blocked by mattresses and chairs, reports the NZ Fire Service.

The New Zealand Fire Service says 40 teenagers were due to stay in the meeting house and an adjacent room on college grounds when the floor of the wharenui caught fire about 8pm Saturday.

The charred remains of the wharenui at Opotiki College. Photos; Supplied.

One of the blocked exits.

Two fire crews arrived to find the building alight and inspectors spent yesterday going through the remains.

Bay of Plenty Fire Risk Management Officer Jon Rewi says tables and chairs had been pushed across some emergency exits and mattresses laid out on the floors – blocking other exits.

The mattresses were placed over under-floor heating vents and as a result of the overheating, Jon says the floor of the wharenui caught fire.

'There does not appear to have been a working fire alarm system – instead a caretaker was alerted when the burglar alarm motion sensor detected the rapidly spreading flames.”

'Luckily the students were out of the two buildings at the time. A few hours later and, without an early warning system, those in the wharenui would have been trapped inside, unable to escape out the exits. There almost certainly would have been fatalities,” says Jon.

He says blocked exits also made it difficult for the first firefighters to get inside.

The fire caused extensive damage to the wharenui.

Jon warns the incident should serve as a warning to other schools and marae to double check their fire safety systems and put in place a careful fire safety plan before hosting overnight stays.

The local fire service can always be asked for advice, he said.

SunLive is currently awaiting comment from the school's principal.