A man has been threatened by a robber who stole his purple Harley Davidson while he sat in Sydney traffic, in what police describe as a rare and brazen theft.

The motorcyclist was riding on Bernera Road, Prestons, when he stopped at a red light at the intersection of Yarrunga Street about 3.30pm yesterday.

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Police said he felt something being pressed into his back as a threat was made.

The man got off his bike and the thief got on and rode off along Camden Valley Way, police said.