UPDATE: Man's body found during search at Hawkesbury River

Police believe they have found the body of a 21-year-old man missing in NSW's Hawkesbury river since Friday afternoon.

The search resumed this morning, with police and State Emergency Services discovering the body at about 8.30am.

The man's body was found north of the Windsor Bridge near the boat ramp.

While he has not been formally identified, police are confident it is the missing Quakers Hill man.

The 21-year-old had been swimming at Windsor Beach around 4.15pm on Friday when he went under and never resurfaced.

Friends say they they noticed the man struggling but were unable to reach him before he disappeared.

Police and the SES spent all day yesterday scouring the water using helicopters and divers.

A report is being prepared by the Coroner.