A truck has killed two people working on a broken-down car beside the Hume Highway south of Sydney. The truck hit a man and a woman south of Mittagong shortly after 12:30pm (AEDT). Police say a tow truck driver aged in his 40s had gone to the aid of a woman after her car broke down. Both died at the scene. The truck driver is being treated in hospital for shock and has undergone blood and urine tests. All three southbound lanes of the highway are being closed intermittently for the police investigations, and there are still significant traffic delays.