Two offenders forced a black bag over the driver's head before piercing his neck in an assault that left the driver severely emotionally traumatised.

A Victoria Police spokeswoman said one of the off enders, thought to be in his 20s, got in the front of the taxi in Richmond about 10.50pm on Saturday and asked the driver to head to Yarra Boulevard to pick up his friend.

When they arrived at Yarra Boulevard, a second man got into the back seat.

That man placed his hand over the victim's face before placing a black bag over his head.

A syringe was then pushed into his neck before the front seat passenger demanded the driver hand over cash.

The pair ran from the taxi with the driver's takings for the night.

The police spokeswoman said the victim suff ered needle stick injuries and severe emotional trauma. He is undergoing medical tests to determine whether he has contracted an illness.

The spokeswoman said the man's head was covered and he could not see whether there was a liquid in the syringe at the time.

Police have released a photo of a man they would like to speak to about the assault.

He is aged in his 20s, of Indian appearance with a slim build and was wearing a dark cap, green shirt and sunglasses.

Anyone with any information about the attack has been urged to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au