M1 lanes reopened after serious crash

All northbound lanes of the M1 Pacific Motorway north of Sydney have reopened following a serious accident just north of the Hawkesbury River Bridge.

Four people were injured when a car driving down the wrong side of the highway crashed into another just north of the bridge at 8.30pm on Wednesday.

A man who was driving down the wrong side was rushed to Gosford Hospital in a critical condition, while a woman travelling in the car he smashed into was airlifted to the Royal North Shore Hospital with two broken legs.

Two other passengers in that car were taken to the same hospital.

A fourth person in their car wasn't injured in the Wednesday evening collision.

All northbound lanes were reopened shortly before 3am on Thursday.

Police are investigating the crash.