The body of a man has been found in a weir in a New South Wales national park after a car plunged into the water in the early hours of the morning. 

Emergency services were called to the Audley Weir in the Royal National Park, south of Sydney, about 1.15am on Wednesday. 

Officers were told a blue Audi sedan and a silver Mercedes sedan had been driving down Audley Road, near Riverside Road, before the blue vehicle came off the road, entered the water and sank.

It's understood the driver, a man in his 20s, was able to free himself from the sinking vehicle and swim to safety. He was taken to hospital for mandatory testing.  

His passenger, also a man in his 20s, was trapped in the vehicle.

Police then recovered the body of a man about 10.20am. He is yet to be formally identified. A report will be prepared for the coroner. 

The driver, and an 18-year-old man driving the Mercedes, underwent a roadside breath test, which returned a negative result.

The teenager has also been taken to hospital for testing. 

The Audi was recovered from the water, with a crane lifting it from the weir on Wednesday morning.

Police have established a crime scene and specialist officers have started an investigation into the incident. 

Motorists have been told to avoid the area, with Audley Road closed in both directions through the morning.