A baby, sitting on his mother's lap without a seatbelt, was critically injured when the van they were travelling in smashed into the back of a parked truck in Sydney's south-west, police say.

The 15-month-old child suffered severe facial and internal injuries, believed to be from an airbag being released, police said.

Message 0424 SMS SMH (+61 424 767 764) or email us with information or images.

The boy's 30-year-old father crashed the Toyota Hiace into the rear of the truck on Bowden Street, Harris Park, about 5am today.

The mother, 26, was trapped in the van for about an hour.