YES, it's a plane if you believe the experts.
The pictures were captured by North Richmond photographer Lindsey Creswell and her husband Mark at their home on Monday of the June long weekend.
Experts think the photos may be the result of a trail of vapour left from a high flying aeroplane, which has been backlit by the setting sun, but others have suggested space junk or even a UFO with a metallic object (circled) in one of the photos.
Mrs Creswell said it was around dusk when she spotted the phenomenon.
"I was looking straight up Bells Line of Road toward Kurrajong when I saw what looked like a sparkling kind of star in the cloudline above the mountains," she said.
"Then it looked like it broke through the clouds and started to descend, leaving behind what looked like a blazing trail.
"It lasted for about 40 minutes. We got in the car and drove about 20 minutes up Bells Line of Road but by then it had become only a faint line."
Having no idea what they'd witnessed but aware they had some great photos, the Creswell's asked The Gazette for some help in getting to the bottom of the spectacle.
Sydney Observatory curator, Dr Nick Lomb, was quick to identify the cause of the evening show, saying the photos showed the vapour trail of the aeroplane, with the twin exhaust emission from the plane's two engines clearly visible thanks to the high resolution photos.
Dr Lomb then directed The Gazette to his colleague Geoff Wyatt's blog on the phenomenon.
"Contrails, condensation trails or vapour trails are all the same stunning phenomena visible in the sky caused by aeroplanes," Mr Wyatt wrote.
"As hot water vapour leaves aeroplane engines at high altitudes it comes into contact with the cold atmosphere and pushes it past the vapour point. The vapour then condenses into small drops of water and can freeze, the effect of which is to create a cloud that trails the plane.
"If the trails are seen around the time of sunrise or sunset they can take on a wonderful golden colour and if seen off in the distance making them look low near the horizon they can look just like the common misconception of a comet."
Dr Lomb said backlit vapour trails can be very spectacular and are commonly seen whenever the conditions in the atmosphere are right for vapour trails to form.
UWS Associate Professor Miroslav Filipovic, a specialist in Computational Astrophysics, Imaging and Simulation at the university's School of Computing and Mathematics, agreed with Dr Lomb's observation, but also put forward the possibility the photos could depict some sort of meteorological balloon used to collect weather data. "But this would be an educated guess given I've only got the photo to base my response on," he said.
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