Three people have died in a crash in Sydney after a man fled police who had been attempting to carry out a welfare check.

A 42-year-old man fled in a white Audi after officers were called to a welfare check at a property in Gymea Bay, southern Sydney, at 12.25am on Saturday.

Shortly afterwards, police received reports that an Audi had rolled over a concrete barrier and landed on a blue Toyota sedan at Sans Souci, about 7km away.

The man died at the scene, as did the driver and passenger of the Toyota, who are yet to be formally identified.

Superintendent Graham Hibbs said police had been called to do a welfare check before the accident.

“All I can really say at this stage is we were called for a concern for welfare, a mental health incident,” he said. “And that’s all the detail I can provide at this stage.”

Police were not able to speak with the man before he fled. Officers believed the Audi was travelling at “considerable speed” when the crash occurred, Hibbs said.

“It was a horrific scene. We really feel for the families of all three persons and their loved ones,” he said.

“I don’t think there were many cars on the road at that time.”

Between 20 and 40 bystanders attempted to assist when one of the cars caught fire.

“We really feel for the families of all three persons who were involved in this incident and their loved ones,” he said.

A crime scene has been established, and investigations are continuing into the crash circumstances, with drugs and alcohol a line of inquiry.

It has been a deadly 24 hours on Australian roads after a 25-year-old woman and an infant died in a two-car crash near a popular holiday spot in Queensland.

In a separate crash in Queensland, two girls, aged 13 and 14, suffered serious injuries after they were hit by a car while on a footpath in South Mackay.

– With AAP