The man, a Chinese national, was swimming with friends in the river at Bendeela when he disappeared underwater about 3.40pm and did not resurface.

Police said the man was on an inflatable mattress before he disappeared.

Emergency services, including police divers, launched a search and recovered the body of the man at about 8.40pm. The body is yet to be formally identified.

The death was the second suspected drowning in the space of several hours on Sunday, as thousands of people took to waterways across the state to battle sweltering heat.

A 25-year-old Nepalese national disappeared underwater while swimming with friends at Bents Basin in Sydney's south-west about 11.30am on Sunday.

His body was recovered about two hours later.

Seven emergency crews and a helicopter were called to the scene, but the only person who was treated was one rescue officer, for heatstroke.

Two other men who were rescued from the water after being seen struggling did not require any further treatment.

Ahead of the Australia Day long weekend, Surf Life Saving NSW issued a warning for swimmers to take care in waterways and swim in patrolled areas.

"Surf Lifesavers will be out in force but sadly we have already experienced at least 13 drownings this summer," Surf Life Saving NSW chief executive Steven Pearce said on Friday.

"Therefore we urge everyone to behave responsibly around the water."

Swimmers have also been urged to drink plenty of water and reapply sunscreen regularly.