NASA issued an alert on Thursday saying a seven-metre, three-tonne, cylindrical object would plunge to Earth over Victoria and Tasmania, said Professor Brian Schmidt, an astronomer at the Research School of Astronomy and Astrophysics at the Australian National University.

“Orbits of these [pieces of space junk] is monitored quite closely," he said. "This one was decaying rapidly and the prediction of the path was confirmed, because everyone saw it."

The object was the "third-stage" of the Soyuz rocket used on July 8 to launch Russia’s second Meteor-M weather satellite, quashing beliefs it was a meteorite.

Residents of Cobar, in western NSW, reported hearing a sonic boom after a five minute delay, which indicated the object disintegrated within 100 kilometres of the town, said Professor Schmidt.

Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, tweeted that the object was: "Object 40077, 3rd stage from Meteor-M launch, reentered over Australia at about 1145 UTC Jul 10".

The fireball has been described as a "massive shooting star" by witnesses across NSW and Victoria, including Dubbo. It was also spotted by residents in Tasmania and South Australia.

Dubbo residents were among the many who saw the 'fireball' on Thursday night, with reports on social media. 

Callers to talkback radio stations reported seeing a bright object travelling east to north about 9.45pm. Some callers said they initially thought it was a burning plane.

Radio station 3AW listener John said he pulled over while driving in Keilor Park to watch the light show.

“It was really impressive,” he said. “It had the flame and the intense burn. Just as it was falling away it broke up. I’d say it was a little asteroid or a comet.”

Professor Brian Schmidt, an astronomer at the Research School of Astronomy and Astrophysics at the Australian National University, told excited Twitter users the fireball was likely space junk, pointing to a 'decay prediction' map he had sourced.

There were also reports it was visible in Tasmania, South Australia and Queensland.

The fireball was a “very rare and exciting” event, said Dr Nick Lomb, curator of astronomy at Sydney Observatory.

He believes the object was space junk because it was travelling slower than the escape velocity from Earth, which is 11.2km/sec or 40,000km/hour.

“Some people saw it for a 10 seconds or more, which is a very long time for a piece of rock from space, which suggests it could have been space junk,” he said.

He said the bright object most likely plunged into the ocean north of Brisbane, since it was travelling in a north-north-east direction.

The Bureau of Meteorology, which received a report of the light show from Bendigo, 160km north of Melbourne, just before 10pm, said it could be a meteor or a piece of space junk.

“I don’t know if anything was expected to re-enter the atmosphere tonight,” senior forecaster Terry Ryan said.

“Obviously something has happened. Maybe it’s a bit of space junk or it could be a meteor. It was described as fairly bright.” 

The Astronomical Society of Victoria said the object could be a meteorite, or it may be a satellite or parts of a satellite reentering the earth’s atmosphere.

President Ken Le Marquand said he received a call from a man at the Queen Victoria Market just before 10pm who reported seeing a meteor going from “horizon to horizon”.

“He said it didn't get quite as bright as the moon and was mostly white with a bit of red at the end," Mr Le Marquand said.

"He said it went for 30 seconds, which is an awful long time. Usually the ones we see in the sky are the size of dust and only last a second. But if you get something a bit bigger, like the size of a pea, it can put on a spectacular light show.”  

Peter Gibson, of the Civil Aviation Safety Authority, said he had received reports from people in Sydney who said they saw a meteor.

Mr Gibson said there had been no reports of aircraft problems. 

Dr Duffy expects dozens of people will scour the surrounding areas of Cobar for bits of debris. Any remnants would likely be the size of dinner plates.

“Depending on how tough the space junk is, there will be pieces on the desert floor. I reckon this is going to be a tough find though,” he said. “There are even apps for this, like one called Fireball, which can help you track it down to hundreds of metres.”