An unidentified flying object that was seen in Sydney's skies had some fearing it was an alien invasion or a missile attack. 

But no, it wasn't a meteor, missile, or a nuke - it turns out it was an unmanned Chinese rocket launch shrouded in secrecy.

About 6:20pm on Tuesday (local time), a boomerang-shaped flare appeared in the sky. It was later revealed to be a Long March 3B rocket, which blasted off from China late on Tuesday.

The rocket's trajectory took it from central China across north and eastern Australia. It was believed to be carrying an Earth observation satellite.

"It most definitely could be in the realm of spy or military satellites," astrophysicist Brad Tucker told 7 News.

The rocket is now in orbit about 40,000km above Earth.

Earlier this year, remnants of another Chinese rocket - the Long March 5B - re-entered Earth's atmosphere and landed west of the Maldives.