A partially paralysed man was threatened with a syringe and robbed on a Sydney train.

Then he caught another train to report the robbery to police.

The man, 53, was on a train from Town Hall to Bondi Junction between 9am and 10.10am yesterday, when four men approached him as the train reached Kings Cross.

One of the men threatened him verbally and demanded money, while another threatened him with a syringe.

He handed over his wallet, with a small sum of cash, got off the train, and travelled back to Town Hall station.

There, he reported the attack to police.

He was not hurt.

Acting Inspector Mick Kelly from City Central police said police were waiting to review CCTV footage to identify the men. He said the man who was assaulted was ''mainly shaken up, but had no physical injuries.''

The man with the syringe is described as being Caucasian appearance, thin build, acne on his face and wearing a blue long sleeve jumper and black shorts.

The other three men are described as being Mediterranean/Middle Eastern appearance with short dark hair. Two had athletic builds, while the third had a larger build.

Anyone with information is urged to phone Crime Stoppers on 1800 333 000.