Man, 21, burned in Sydney gas fireball

A gas-fuelled fireball in a busy northern Sydney shopping plaza has left a man with serious burns.

The 21-year-old was rushed to Sydney's Royal North Shore Hospital with burns to his lower body after a gas main caught alight about 1pm (AEST) on Monday at the front of a shop in Lane Cove.

He was reported to be in a stable condition.

The local primary school was locked down and about 150 people were moved to safety as firefighters monitored the fire.

They decided not to put the fire out, opting instead to ask the gas company to shut the gas down - which it did an hour later.

"It's actually safer to allow the gas to burn," Superintendent Ian Krimmer told AAP.

"If you put the fire out, you create a bigger problem because the gas leak could go to other areas and cause explosions in other locations."

As the gas was being shut off, six fire crews were protecting buildings, while police kept Longueville Road closed to all traffic.