Just after midday, the cars collided and crashed into trees on the median strip, causing lengthy holiday traffic delays as emergency crews rushed to the scene.

The female passenger of a Toyota was taken to hospital in a serious condition while the male driver of the car, who was uninjured, underwent mandatory testing under police guard.

Police said the man driving the other car suffered serious leg injuries, while his male and female passengers received minor injuries. They were all taken to Goulburn Hospital.

Emergency crews remained at the scene as of 3:15pm and police are still investigating.

One southland lane closed on the Hume Highway near Marulan was not reopened until almost 5pm, with delays stretching back more than 20 km.

Traffic remains heavy and drivers have been advised to take care and allow extra travel time.