Two people have died and another man is under police guard in hospital after a terminated pursuit involving an allegedly stolen car ended in a devastating crash in Sydney’s south west.

A critical incident investigation is now under way into the circumstances surrounding the pursuit and collision.

Police allege a 25-year-old man was assaulted by an unknown man at a service station on King Georges Rd in South Hurstville about 10.20am on Saturday. He then allegedly stole the man’s red sedan and fled.

Officers later spotted the vehicle on King Georges Rd in Beverley Hills and directed the driver to stop.

A pursuit began along the M5 motorway, with Polair and multiple police commands assisting. The chase was terminated in the Bankstown area.

Polair continued tracking the vehicle from the air before it collided with an Alfa Romeo on the Old Hume Hwy at Camden South about 11.10am.

The two occupants of the Alfa Romeo were treated by paramedics but died at the scene. They have not yet been formally identified.

Police allege the driver of the allegedly stolen vehicle fled on foot after the crash but was arrested a short distance away.

He was taken to Campbelltown Hospital for mandatory testing and is expected to be taken to Campbelltown Police Station for questioning. No charges have been laid.

Multiple crime scenes have been established, with a critical incident team leading the investigation into the alleged car theft and crash. The probe is being supported by St George Police and the Crash Investigation Unit.

The matter will be reviewed by the Professional Standards Command and overseen by the Law Enforcement Conduct Commission.

The Old Hume Hwy remains closed in both directions, with police declaring the area a crime scene. Motorists are being urged to avoid the area.