Four men on Harley-Davidsons have held up a motorcyclist at traffic lights in Sydney, threatening him with a gun before stealing his souped-up bike, with NSW registration SVAGE, police say.

The 32-year-old man was riding his black 2008 model Chrome Horse chopper in Milperra about 9.30pm yesterday when he stopped at a set of lights on Bullecourt Avenue.

The four men, riding two Harley-Davidsons, pulled up on either side of him, pointed the gun and demanded the bike, police said.

Police said one of the men got on the bike and all four rode away towards Henry Lawson Drive.

All of the men had their faces covered and police have not been given a description of them.

Inspector Bill Lardner of Bankstown Local Area Command said investigators were still looking into the motive for the theft.

He said he was not aware of similar crimes in the Bankstown area.

Heavy Duty magazine technical editor, Doc Robinson, said the motorbike was a high-powered custom chopper.

A 2008 model would probably be worth about $25,000, depending on its customised features.

"[Chopper] is a broad term, but usually they're long bikes with forks that go out a long way," Robinson said.

He said a chopper was the type of motorcycle ridden in the 1969 American film Easy Rider.

Christopher Burns of the Motorcycle Council of NSW said it was not clear what the men would do with the stolen chopper, but that breaking up the bike and selling its parts was possible.

"It could have $10,000 of performance parts on it," Mr Burns said. "The average sports bike, if it was in a crash, could have replacement costs of around $4000."

A total of 1871 motorcycles were reported stolen between July 2010 to June 2011 in NSW, the National Motor Vehicle Theft Reduction Council (NMVTRC), which publishes statistics on motor vehicle theft, said in its 2011 annual report.

In all states, there were 7695 reported thefts, the NMVTRC added, down from 7916 thefts between July 2009 to June 2010.

Police asked anyone with information to contact Bankstown police or Crime Stoppers on 1800 333 000.

- with Glenda Kwek