A man is in a critical condition after being hit by a car in Sydney's south around lunchtime on Monday.

Emergency services were called to the corner of the Princes Highway and Forest Road, Arncliffe, just after 1.15pm.

Four NSW Ambulance units arrived at the scene to find the injured man, 43, unconscious with "major head injuries".

He was treated at the scene and intubated, before being transported to St George Hospital, where he remains in a critical condition.

The male driver was treated for shock and taken to St George Hospital for mandatory testing.