Sex workers are being threatened and hurt by gang members in Hawke's Bay for refusing to pay a tax for working.

Police are investigating two incidents where a 38-year-old Napier man demanded prostitutes pay him money to 'be in business'.

The man will face charges in court next week of inducing an indecent act and assaulting a female.

Detective Jeff Foley says the man told the sex workers he was in Black Power and she had to pay him money to work, which is completely illegal.

He says they're concerned about the levels of violence these men are using against sex workers.