Police are looking for a man who sprayed a substance, believed to be pepper spray, into the face of a bus driver.
Officers said the suspect was told in Surrey Hills, Sydney, that he could not board the bus as it was full.
The man, who is said to have spoken with an English accent, can be seen verbally abusing the driver before spraying a chemical into his face.
Video released by New South Wales police shows the suspect arguing with the driver on Thursday evening, then sitting on the dashboard of the bus and carrying out the assault.
Paramedics were called when the driver, a 41-year-old man, began having trouble breathing.
CCTV of the incident has been released in a bid to identify the man involved.