A male police officer and the female driver of a second car are fighting for their lives in hospital after a head-on collision in Sydney's south on Thursday.

Emergency services were called to New Illawarra Road, near Heathcote Road in Lucas Heights after reports of a serious multi-vehicle crash about 2.10pm.

Emergency services at the scene of the crash. Credit:Nine News

According to Nine News, the officer, a man in his 30s, was returning to Liverpool police station in his own car after a training exercise when one of the cars crossed to the wrong side of the road.

A spokesman for NSW Ambulance said it took "some time to get access" to the drivers and extricate them from the smashed vehicles.