Police divers have found the body of a 23-year-old man who was seen struggling in the water after jumping off the Como Railway Bridge in Sydney's South earlier today.

The man and his two friends had been jumping from the bridge into the Georges River before emergency services were called to the scene at 1:35pm after multiple triple-zero calls.

NSW Police inspector from Sutherland Local Area Command Steve Worthington said the man had landed in the water awkwardly and did not resurface.

NSW Ambulance Chief Inspector John Brotherhood said the man was spotted struggling in the water.

"He apparently came up for a brief time onto the surface and was in difficulty and went down again," he said.

Divers found the man's body after 4:00pm.

A local man, Josh, was approaching the bridge to refuel at a petrol station nearby when he saw the three men about to jump.

"As we pulled in they jumped and I heard a splash … and as I was paying for fuel the two mates ran inside and said, 'He hasn't come up, he hasn't come up!'

"One of them was white, he nearly passed out."

Josh said at this time of year jumping off the bridge was very popular among young people.

"They're young, just trying to have fun, what do you do?" he said.

"They didn't think before they jumped … they were filming it … just having a laugh."

Chief Inspector Brotherhood said that jumping off the railway bridge was a very dangerous pastime.

He advised people to check for unseen debris, branches and rocks before jumping into any body of water.