Police have released CCTV footage they hope will lead to identifying the man allegedly responsible for spraying an inner Sydney bus driver, in what a detective has described as the "despicable behaviour" of a "coward".

The 41-year-old bus driver was allegedly sprayed six times in the face with the chemical, believed to have been capsicum spray, while driving a full bus in Surry Hills on Thursday night.

CCTV shows the moment police allege a man attacked a bus driver with pepper spray. Credit:Police Media

The alleged assault occurred on board the Route 392 bus as it travelled down Oxford Street towards the city, stopping between Flinders Street and Crown Street in Surry Hills about 10.40pm.

Police allege the male offender was told he could not get on the bus, which was at capacity, "due to safety reasons".