There is large scale human trafficking occurring in some licensed Victorian brothels, Victoria Police have told a parliamentary inquiry.

While the majority of brothels are not using trafficked sex workers, there are some where trafficking is occurring, the federal inquiry into slavery and human trafficking heard.

But police admit they do not know the scale of trafficking in Victoria's illegal brothels and cannot say how many illegal brothels there are.

The cloak of anonymity and secrecy surrounding the industry makes it hard for police to investigate, Senior Sergeant Marilynn Ross told the inquiry.

She said most of the approximately 100 licensed brothels in Victoria did the right thing.

"But we suspect that in a small number of those licensed brothels human trafficking is occurring and it is occurring on a quite larger scale," Sen Sgt Ross said.

"It is not that we go in and we suspect one person in there that might be trafficked. We suspect that there is a few in that particular brothel."

Sen Sgt Ross said people often got into slavery situations because they owed money to people and were brought to Australia to become a sex worker to pay off the debt.

Another officer Detective Superintendent Rod Jouning said it was not possible to say how many illegal brothels there are because police cannot just walk through the doors to investigate.

"We're not and never will be in a position where we go in and engage in sexual services to obtain that proof," he said.

Earlier the inquiry heard an increasing number of Australian businesses were making sure their products adhere to fair trade principles.

Christine Carolan from Australian Catholic Religious Against Trafficking in Humans said Australians are horrified when they find out people's human rights are being violated for their goods.

She said companies were interested in being socially responsible.

"Human rights are never negotiable ... I think we have got to be very clear about that," Ms Carolan said.