School blast injures lollipop man

A gas explosion at a school in Sydney's southwest has seriously injured a lollipop man and forced the evacuation of more than 500 students.

Emergency services say the 50-year-old school crossing volunteer at Unity Grammar College in Austral, was storing his stop sign when the blast occurred near the canteen about 9am (AEDT) on Wednesday.

Students had already begun classes but none were hurt.

The well-liked staff member, who has also worked as a night watchman at the Islamic school which opened two years ago, had stepped into his storage room only moments before the explosion.

Still conscious and able to speak, he was treated by fire crews treated until paramedics arrived.

He was flown by helicopter to the Royal North Shore Hospital with burns to 60 per cent of his body and was reported to be in a serious condition.

Inspector Siobhan Busetto said it was a case of being in the wrong place at the wrong time.

"He had just finished his traffic management duties and was returning his sign," she said.

"Fortunately the children were in class."

The school was locked down after firefighters examined the scene and helped evacuate 530 students who were sent home.

The NSW Fire Brigades said initial investigations indicted the explosion was most likely caused by a leaking gas bottle, but that had yet to be been confirmed.

"We are looking at an accidental gas leak," Superintendent Ian Krimmer said.

"There are no suspicious circumstances."

He said there was no further risk from the gas and a structural engineer would decide whether the building was safe for use.

Principal Walid Ali was relieved no children were injured.

"Our thoughts and concerns go out to the adult victim and his family and we hope he has a very quick recovery," he told Fairfax's Liverpool Champion.

"The staff member that was injured was a very highly regarded member of our school community."

Mr Ali said counselling would be made available for the students but so far they appeared to have responded well to the incident.

The school is due to reopen on Thursday.