The body of a 15-year-old boy has been found at a rocky waterhole in Sydney's south-west, after he disappeared under the water while swimming with friends this afternoon.

Police divers had this afternoon been scouring a swimming spot in Keith Longhurst Reserve, at Kentlyn near Campbelltown, after emergency services were called to the Georges River scene just after 2.30pm.

The teen was reportedly swimming with three friends, in what residents have described as a notoriously dangerous swimming spot, when he failed to surface.

Police divers found his body about 5.10pm. He has not yet been formally identified.

Laura Bentham, the mother of one of the friends swimming at the waterhole at the same time, said she feared the worst when she heard the news.

"I messaged my son around 3pm when I heard the news - because he is also 15 - I just lost it. Thankfully he messaged me back saying he was okay but that's all we heard. We don't know who the victim is yet," she said.

A resident said the swimming hole was notorious for accidents because a rocky ledge lies about a metre below the water and cannot be seen from above.

"There have been five or six accidents at the waterhole in the past few years," he said. "I think the boy may have jumped and hit his head."

Five NSW Ambulance crews and a Toll Rescue Helicopter specialist medical team are on scene searching for the boy.

Police have blocked off a road near the reserve as police divers scour the waters in search of the boy.

Originally published as Body of 15-year-old found at Georges River swimming hole