The pictures, snapped in the northern suburbs, appears to show a bright object drifting through the sunset.
The UFO has a thick and short tail, ruling out the possibility of a meteorite. Others speculated the item could have been space junk making a fiery re-entry.
A still of the UFO taken from a video shot by Quentin Theron from his Coconut Grove home. Picture: Quentin Theron
Quentin Theron, 40, said his two kids, Kylie, 14, and Georgia, 8, were in the pool at their Coconut Grove home when they yelled out they could see a shooting star about 6pm Monday.
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“I said ‘that’s crazy. It’s too early and it’s too bright for a shooting star’,” he said.
“My son said it was a meteorite.
“I said I think it was a satellite that had had its life. We couldn’t decide what it was.”
Minas Kassiou shot this photo of the same UFO over Darwin. Picture: Minas Kassiou
Astronomer Geoff Carr said even he was baffled by the photo of the object.
“I can only suggest a high altitude military jet with its afterburner on,” he said.
“Other than that it looks too slow to be a piece of space junk re-entering the Earth’s atmosphere.
This is a still of a fast-moving UFO filmed by the Territory’s most well-known UFO spotted, Alan Ferguson, over Acacia Hills. Picture: Alan Ferguson
“It may be an Iridium Flare – which is a large, cube-shaped communication satellite.
“These appear to flare up sometimes extremely brightly if at the right angle to catch the sunlight.”
But Mr Carr – who has been an astronomer for more than 20 years and operates his own Star Safaris business – stopped short of claiming that the UFO was alien.
“It is unidentified, flying and an object, but I don’t think it is of alien origin,” he said.
“Other than that I have no idea – it’s certainly an interesting piece of footage.”