A man has been stabbed in a bus queue outside Wynyard Station in the Sydney CBD. 

Police said the man was stabbed in the back of his neck by a second man while waiting for the bus on Carrington Street about 2pm. 

The victim was treated by paramedics at the scene before being taken to St Vincent's Hospital in a stable condition.

Witnesses said the man may have used a card to slash the man's neck. 

"This man was standing in the queue like a regular normal person, just trying to get home for the day, and this man just runs up straight behind him," a woman told 9NEWS.

"It looked like he was going to tap him on the shoulder, like they knew each other, but he actually he had some kind of Opal card or card and he started stabbing way at this man's neck and ran off."

The alleged attacker, who police said had a "noticeable boil" on his face, fled the scene but was arrested about 3pm nearby.

He has been taken to the Day Street police station for questioning.

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

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