Two people have died in a light plane crash on the New South Wales mid-north coast.

The plane came down alongside the Pacific Highway near Kew, south of Port Macquarie, around 4:00pm on Saturday.

Police attended the crash site and located the bodies of a man and a woman.

No other passengers were on board.

Tony Hill, who was travelling southbound on the highway, witnessed the crash.

Mr Hill, who is from the Central Coast, said he was less than a kilometre from where the plane came down.

"I saw a light aircraft banking sharply and completely turned on its side. [It] went completely over and went into a bush area about 15 metres from the highway," he said.

"I knew something catastrophic was happening because of the way it was banking.

"It basically banked on its side as it sort of swept over that tree line on the other side of the highway and then it sharply rolled onto it's back and then came straight down.

"It was quite frightening to see actually. And there was no explosion, which was amazing, as it hit that tree line and basically disintegrated."

A crime scene has been established and will be examined by specialist forensic officers.

An investigation into the cause of the crash will be undertaken by the Australian Transportation Safety Bureau.