Police said the 23-year-old man was driving west along Port Stephens Drive, near Nelson Bay Road, in Salamander Bay just after 11pm on Monday when his vehicle veered off the roadway and crashed into a tree.

The man, who was the only person in the vehicle, was critically injured and died at the scene.

Police from the Port Stephens local area command are investigating the circumstances surrounding the man's death.

About three hours later, police were called to a separate crash involving a 16-year-old driver in Moss Vale in the Southern Highlands.

Police said the teenager was driving along Berrima Road, near Brookdale Road, just before 2am when his vehicle left the road and crashed into a power pole.

The teenager initially was trapped inside the vehicle by live power lines that were brought down in the crash, restricting access for his rescuers.

The power was turned off and the teenager was freed from the wreckage and flown to Liverpool Hospital in a serious condition. His injuries are not life-threatening, police said.

The minimum age for a person to obtain their P-plates is 17. Police could not immediately say whether the teenager was licensed, or whether he was being supervised at the time of the crash.

Earlier in the evening, police were called to Merrylands in Sydney's west after a vehicle being driven by an 18-year-old man mounted a gutter and clipped a six-year-old boy.

Police said the vehicle, which was displaying P-plates, was preparing to turn into Merrylands Road from Railway Terrace about 6.30pm when it left the roadway and clipped the six-year-old boy, who suffered cuts and bruises.

He was taken to hospital for precautionary medical tests, but his injuries were not thought to be serious.

Police said the driver was breath-tested at the scene and returned a negative result.

Police are appealing for anyone who saw any of the crashes to call Crime Stoppers on 1800 333 000 or use the Crime Stoppers online reporting page.