Sinkhole forces residents to evacuate in Illawong, Australia

The sinkhole in Illawong that opened up about 4pm on Monday.

More than a dozen people have fled their homes at Illawong in Sydney's south after a sinkhole swallowed a backyard on Monday afternoon.

The three-metre deep hole opened up about 4pm, forcing police and firefighters to evacuate a row of townhouses on Osprey Drive.

One man was asleep on his couch when he heard a loud crack in his garden outside, the man's friend, Hisham Bashir said.

"He looked outside and the tree had collapsed and the ground started to sink," Mr Bashir said.

"Dirt had sprayed up all over his backyard and on the trampoline. There was dirt all over the windows.

"It's pretty scary, but these things happen," he said.

Police said the hole measured 10 metres by six metres, and was three metres deep. They said it opened up without warning.

Thirteen residents inside the homes were evacuated as a precaution.

They were allowed to return to their villas after structural engineers attended the scene and declared the buildings safe at about 9.15pm.

The townhouses sit on top of a massive garage that services the 22-unit complex, which is prone to flooding, most recently during the Sydney storms in April.

The State Emergency Service said it suspected the flooding had leached into the ground out the back of the property, causing the backyard to cave in.

The site has been cordoned off and police are waiting for power and water engineers to inspect the damage.

Emergency services said it was unlikely affected residents would be able to return for a couple of days.