A plane was forced to swerve to narrowly avoid crashing into a mystery object near Perth.

The Australian Transport Safety Bureau has revealed details of the near collision, involving a small aircraft at 9.13am on March 19, in a report released yesterday.

In the report, investigators say it is not known what the object was or how it came to be in military restricted airspace north of Perth.

The De Haviland DHC-8 charter plane carrying four crew was heading to Perth Airport from Kambalda when the pilot spotted a bright strobe light about 23km north of Perth, 3,800ft above sea level.

Crew saw the light was coming from a grey cylindrical object and appeared to be heading towards the plane.

The report says the pilot was forced to turn to avoid the object and passed with just 20m spare.

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Investigators say the Australian Defence Force was not operating any unmanned objects in the area at the time.

Meanwhile, three days later, a the pilot of a Westpac Rescue Helicopter in Newcastle, New South Wales, reported seeing a “white light” which appeared to be following the helicopter.

Investigators said the light likely belonged to an unmanned aerial vehicle.