A man who believes he found a piece of space junk from the Russian rocket which spectacularly re-entered the atmosphere over Australia put it up for sale for $100,000.

Last week's fireball known as Object 40077 was the third stage of a Soyuz rocket which was launched from Kazakhstan on July 8.

It plummeted to Earth at 29,000 kilometres per hour on Thursday night.

Many people reported seeing a bright object travelling east to north about 9:45pm on Thursday.

The man, known only as Peter, said he found the globe-shaped object just north of Walgett in New South Wales on Sunday.

He believes the object is a fuel cell from a Russian rocket. He posted the object for sale on Gumtree but the ad has since been removed.

Seller "Peter" believes the object is a fuel cell made of titanium and he hopes to get $100,000 for it. ( Gumtree )

In the advertisement, Peter said the object was found on the Castlereagh Highway, under the Pagan Creek bridge.

He said it was made of titanium, 40 centimetres in diameter and weighed 13.4 kilograms.

"One of the valves is completely burnt off and has left some spectacular flow lines of melting titanium as it re-entered earth's atmosphere," Peter said in the Gumtree advertisement.

"The second valve is still partially intact."

The seller did not claim it definitely came from the space junk sighted over Australia last week but said "it certainly appears that way".

Researcher says item 'certainly looks' like space junk

A member of a team of Australian researchers currently tracking the flight path of the Soyuz rocket told the ABC the item "certainly looks" like space debris.

Professor of Planetary Science Phil Bland from Curtin University in Western Australia said the item is probably from a space rocket.

"These types of fuel pods can be one of the most resistant [to the high temperatures of re-entry] things people find," he said.

However he doubted it is from the Russian rocket that was seen over Australia last week.

He said where it was discovered, near Walgett, is hundreds of kilometres east of the path of the re-entry.

"What is a bit surprising is the track it took, which is long, long away from the line we got from the US and most of the eyewitnesses," he said.

"You may get things scattered a little bit - say plus or minus a few kilometres - but there is no way it can move so far."

He said experts believe the item being offered for sale is from a previous rocket re-entry.

'Fireballs in the sky' app aims to help researchers

Researchers have asked people to share images or video they may have captured last Thursday night.

Professor Bland said the most northerly report so far is from Moreland Downs, a station about 240 kilometres west of Bourke, in far northern New South Wales.

He said from there the trajectory of the rocket was into outback southern Queensland, but there have been no reports from that area as yet.

At the moment debris could be anywhere in a "ridiculously huge area", he said.

If it can be narrowed down to an area of two or three square kilometres a search for debris may proceed, involving researchers from Western Australia, Victoria and New South Wales.

"No-one has tracked debris, determined a fall position, and recovered it before," Professor Bland said.

"There are useful data we can get from folks, even if they don't have a photo or video."

Professor Bland's team has developed a smartphone app - 'fireballs in the sky '- that lets people record all the details of a fireball that the scientists need to work out where something landed.

"If you can remember roughly where it was in the sky, download the app and go through the steps to record it," he said.

In 1979 remnants of the USA Skylab space station hurtled to earth from orbit and scattered its remains across the Australian outback.

Look back at how social media reacted to the object that lit up eastern Australian skies: