Nineteen people were injured when a pick-up truck smashed into a cafe in northern Australia, sparking a fire that engulfed the building, police said.

Officials were trying to determine what caused the driver to veer off the road and into the Serves You Right Cafe in Ravenshoe, south of Cairns in Queensland state.

The crash caused a gas bottle at the cafe to explode, setting off a fire, Queensland police spokesman Todd Saunders said.

Three people suffering serious injuries were airlifted to a hospital in Cairns, Mr Saunders said. The others were being treated at local hospitals. The driver was among the casualties.

Queensland Ambulance Service Assistant Commissioner Rod Sheather said patients were being treated for a "varying degree of burns".

Kate Lewis, who works in a supermarket across from the cafe, described the scene as horrific.

"We just heard a massive sonic boom and went running out and saw the cafe on fire and lots of burned people," she told the Australian Broadcasting Corp.