Flashes of light seen across the sky over south-eastern Australia on Monday night were likely to have been remnants of a Russian rocket, the country's space agency said.

The dazzling display was spotted over the southern states of Victoria and Tasmania, captivating stargazers who thought it might be a meteor.

But the Australian Space Agency said it was more likely to have been a Russian Soyuz-2 rocket re-entering the Earth's atmosphere.

The rocket had been launched earlier that evening from Plesetsk Cosmodrome in Russia's north-west, the agency said.

"This launch was notified and remnants of the rocket were planned to safely re-enter the atmosphere into the ocean off the south-east coast of Tasmania.

"The Australian Space Agency will continue to monitor the outcomes of this re-entry with our government partners."

There is growing concern about the risks posed by wayward space junk to satellites, humans and the environment.  

Jaime Andres Alvarado Montes of Macquarie University said Earth's orbital traffic had become "congested" with a "rise in objects and subsequent close encounters".