The fire that destroyed the oldest building at a North Shore school has been found to be accidental.

Northcote College closed its doors to students for a week after the more than 100-year-old heritage-listed sports pavilion was destroyed in a blaze and the asbestos-filled building was demolished.

Students have now been allowed to return to the school, which has announced the cause of the fire in an email to parents.

“We are so sad to have lost a heritage building, the oldest building on the school site,” the email read.

“We have been very well supported and are grateful to firefighters, fire commanders, and other emergency services who attended the fire, and to the Ministry of Education, contractors, and our community.

“The fire investigation has found that the cause of the fire was accidental.”

Investigators have found the fire that gutted a building at Northcote College was started accidentally. Photo / Dean Purcell.

The incident came nearly 12 years after the college’s technology block was wrecked by a blaze.

The fire on February 20 took place during a New Zealand Professional Firefighters Union (NZPFU) strike, meaning paid crews could not respond until action had ended.

Fire and Emergency New Zealand said crews were alerted to a fire at 12.17pm and the Silverdale Fire Brigade was the first to arrive on the scene just before 12.30pm.

Northcote College was closed to students while the burned building was cleared. Photo / Dean Purcell

At the time, Fire and Emergency accused the union of “gambling with the public’s safety” and called for future strikes to be called off.

NZPFU vice-president Martin Campbell said the fire was “another failure for Fire and Emergency” and said the delay fell “squarely” on them.

“Despite nearly 19 months of bargaining, [Fire and Emergency] have failed to provide a fair and reasonable offer to NZPFU membership,” he said.

“Volunteer firefighters did an amazing job containing the fire until the arrival of career firefighters with specialist capabilities.

“Our volunteers shouldn’t have to be placed in this position by Fire and Emergency.”