Paul Brian Edward Connelly, on trial accused of trying to kill his two sons in a car fire using gas cylinders, sat in the Supreme Court in Burnie yesterday while his eldest son gave evidence.

The child told the court when Mr Connelly picked him up from after-school care on December 5 last year, there were chips and lollies in the back of the car.

The boy told police he believed his father got the food ``to keep us in the car so we could stay there'', the court heard.

He described to police hearing a whistling noise in the car after they returned home, but said his father told him it was a new sound the vehicle made, the court heard. 

The child told police that when he heard a bang, he thought it was a gas bottle sitting in the boot.

A whooshing noise followed and the car caught fire, the boy told the court.

He escaped through a passenger door, the court heard. 

The child said he did not see his father have a cigarette in the vehicle before the explosion. 

The boy had 13 operations at the Royal Children's Hospital in Melbourne between December 2012 and January this year,  including a skin graft, North West Regional Hospital's director of emergency services Dr Marielle Ruigrok told the court.

His brother needed six skin graft operations at the hospital in that time, she said. 

Mr Connelly and the boys sustained burns to about 30?thper cent of their bodies, the court heard. 

Firefighter John Hardstaff, appearing as a witness, said after extinguishing fire in the car's hatch area he noticed a gas cylinder there was emitting gas. 

He said in 24 years of service he had never attended a fire caused by a gas leak when the gas cylinder was turned off.

Tasmania Fire Service's Rob Deverell told the court a gas cylinder had to be turned on for a leak to occur. 

He said he smelt LPG  at the car fire, the court heard. 

Mr Deverell said he was surprised by how far the fire had advanced when he arrived, and that car fires usually progressed slowly. 

The trial continues today.