Brothels in Geelong will be targeted by the Fair Work ombudsman as part of a state-wide crackdown on the underpaying of workers in the sex industry.

A hundred brothels in Geelong, Melbourne, Bendigo and Shepparton will be checked in a four-month blitz.

Inspectors will ensure managers and receptionists are receiving award rates as well as penalties, overtime and leave entitlements.

The investigation won't cover sex workers themselves, who are employed as sub-contractors.

But the brothel industry says the government would be better off looking into the number of illegal brothels where immigrant women are forced into sex slavery.