Three men are dead after two light planes collided midair before crashing into bushland in Sydney's south-west.

Emergency crews rushed to the scene at Belimbla Park, near Oakdale, at about midday after residents witnessed the crash.

At least five ambulance crews along with police and fire crews are parked nearby, with first responders making their way to the two wreckage sites on foot.

The two crash sites are in close proximity and one plane burst into flames on impact.

Police said three men had been found dead at the scene.

New South Wales Police Acting Superintendent Timothy Calman said one of the planes involved was a Cessna 182 which had been travelling from Cessnock to Wollongong with two people onboard.

The other was an ultra-light aircraft from a nearby airfield at The Oaks and had one man onboard.

Acting Superintendent Calman said witnesses saw "debris coming from the sky" before rushing to the crash site to help.

"Unfortunately, upon reaching that scene they would've very quickly established there's probably not much that could've been done to save the people onboard and that's a sad and tragic outcome," he said.

Acting Superintendent Calman said the collision happened in uncontrolled airspace and both crashes, which were about 1 kilometre apart, were "not survivable".

He said they were still working to identify two of the victims.

NSW Ambulance Inspector Joseph Ibrahim said 10 ambulances were deployed, including a rescue helicopter.

"Unfortunately there was nothing they could've done," he said.