The family was trying to light the barbecue at a picnic area in the Belanglo State Forest about 12.30pm on Sunday when the explosion occurred, injuring three adults and two children, police said.

One adult who sustained serious burns to his face and chest was airlifted to Concord Hospital, where he was in a satisfactory condition on Sunday night, a hospital spokeswoman told AAP.

A second person, a 65-year-old man, was taken by road to Liverpool Hospital, where he was also reported stable on Sunday night.

Inspector John Klepczarek from Bowral Police Station said up to 20 extended family members were present at the picnic when the incident occurred.

"They put some sort of accelerant on the barbecue and it all went up at once like a big ball of fire," he said.

"We are not sure yet why they did it, but it appears the family were crowded around the barbecue area when the explosion occurred."

At least 150 people were in the picnic area when the blast occurred.

Police are still trying to establish the exact cause of the explosion but Inspector Klepczarek said pouring any type of accelerant over a fire is not "the smartest thing to do".

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