Russian rocket 'meteor' metres in size

The space junk that streaked across Australian skies as a light show was a Russian rocket part that would have been metres long, experts say.

Social media websites lit up from around 9.45pm on Thursday with reports of sightings of a bright object with a long tail shooting over Victoria, Tasmania and NSW.

Swinburne University astronomer Alan Duffy said he used the location of tweets about what was thought to be a meteoroid to piece together the object's trajectory.

He then cross-checked with Nobel Prize winning astronomer Brian Schmidt and confirmed it was a piece of a Russian rocket used to launch a weather satellite from Kazakhstan on Tuesday.

"He'd been given a heads-up by NASA that this was coming," Dr Duffy told AAP.

"They'd been tracking it because it was a Russian rocket launched on July 8."

He said the fact it was burning up for so long indicated it must have been "quite massive".

"It had to be something quite a few kilos if not tonnes," Dr Duffy said.

Sydney observatory astronomer Melissa Hulbert said the piece of the rocket probably weighed 100 to 120kg and would have been "metres" in size.

The light show people on Australia's east coast saw would have been a piece of the rocket designed to fall-away as part of the satellite launch, she said.

"It's kind of like the Apollo mission," she told AAP.

"Parts would be dropped off at various points throughout the mission."

Dr Duffy said NASA would have known the Soyuz's rocket was due for re-entry but it would have been difficult to predict where.

"These things move so fast, it literally does an orbit of the earth in 90 minutes," Dr Duffy said.

"If you're out by a couple of minutes it misses Australia and gives Asia a light show.

"We would literally have had to warn the entire world something might happen."

He said it was possible some debris had landed in northern NSW or Queensland.