According to police, the 36-year-old woman disappeared while paddling in the river at Wilberforce at about 2.30pm the day before.

Witnesses to the incident told police they watched the woman swim half-way across the river and duck diving under the water before coming back up several times, until she failed to resurface on the last occasion.

Police and emergency services were contacted and a search of the river conducted with the assistance of Hawkesbury State Emergency Service (SES) volunteers, and an ambulance helicopter.

The search was suspended at 5.30pm after efforts to locate the woman failed.

Police resumed their extensive search early the next morning using police divers, with officers discovering the woman's body three metres below the surface close to where she was last seen.

Police believe the woman may have gotten caught in weeds.

A report will be prepared for the coroner.