Two light planes have been involved in a fatal midair collision in Sydney's southwest.

Emergency crews responded to reports the planes collided about 11.50am on Saturday near Belimbla Park in Oakdale.

Police have confirmed fatalities but have not said how many.

They have set up two crime scenes, and a number of ambulance and fire crews have responded to the crash.

Authorities are urging the public to stay away.

The Australian Transport Safety Bureau will investigate the crash.