'Drunk driver' arrested after Sydney crash

A man has been airlifted to hospital while another has been arrested for allegedly drink-driving following a two-car crash north of Sydney.

Police say initial investigations suggest the cars collided at an intersection on the Pacific Highway at Mt Colah on Thursday afternoon.

A passenger in one of the cars, believed to be aged in his 30s, was flown to hospital with serious injuries, while the driver suffered minor injuries.

The driver in the other car was arrested after he returned a positive breath test, police said.

The accident comes just hours after a semi-trailer overturned on the Pacific Highway near Berowra, about 8km north of the Mt Colah crash.

The 59-year-old driver was trapped beneath the cabin of the semi-trailer for 45 minutes before being airlifted to Sydney's North Shore Hospital in a serious condition.

The motorway was reopened but police have urged drivers to re-think their travel plans, with traffic expected to be back up for hours following the second accident.