Two women and a seven-year-old boy have suffered burns to their legs in Sydney's southwest after a barbecue erupted into a fireball when methylated spirits was used to start it.

The CareFlight medical rescue service said the barbecue exploded at a home in Glenfield on Christmas Day afternoon.

Police, fire, ambulance and CareFlight crews rushed to the home, the rescue helicopter landing in a field 200 metres from the house to allow the crew's trauma doctor to assist ambulance paramedics treating the three.

The CareFlight doctor said the barbecue erupted into a fireball when what is believed to be methylated spirits was used to help start it.

A seven-year-old boy and his mother, from Kellyville in Sydney's northwest, suffered leg burns, as did another woman.

The three were taken to hospital by road ambulance.