'Bomb' found at Sydney electrical substation

Investigators return from examining the device. Photo / News Corp Australia

The bomb squad is reportedly investigating an explosive device that was found at an electricity substation in western Sydney.

Workers at the Kingswood substation on Smith Street found what is believed to be a petrol bomb with a detonator shortly before 9am on Thursday, The Daily Telegraph reports.

Police have confirmed an operation is under way.

Nearby Nepean Hospital told AAP it was not evacuating but was taking advice from NSW Police.

The nearby Kingswood South Public School remains open.

At least one home neighbouring the substation has been evacuated, according to posts on social media.

The device is described as a lunch-box size container full of fuel and a device similar to a mobile phone attached with wires protruding.

HAZMAT and fire crews are on site and police have established an exclusion zone around Smith Street and Fragar Road.

- AAP