An international anti-slavery expert says the problem of human trafficking and slavery in Australia is much worse than official figures suggest.

A recent parliamentary inquiry estimated there are between 300 and 1,000 female sex slaves in Australia.

At a forum in Brisbane today, University of San Francisco spokesman Dr David Batstone said most Australians do not realise the extent of the problem.

He says at another forum on the issue, he heard a report of a restaurant that kept slaves in its basement.

"An audience member said 'I know another restaurant that's doing that right now. What do I do about it?'," he said.

"So it gives me - this is anecdotal evidence, or case evidence - that suggests that it's much broader than the maybe 1,000 or 2,000 that are projected that are probably in Australia."

'Blood on our teeth'

Meanwhile, World Vision says Australians are eating cheap chocolate produced by child labour.

The charity has launched a national campaign against slavery and human trafficking.

World Vision Australia chief executive Reverend Tim Costello says on a recent visit to the Ivory Coast and Ghana he saw the chocolate industry using child labour.

Speaking in Brisbane today he said the industry had been resisting pressure to clean up its work practices.

"We have silly little guilts. We say 'oh, I sinned, I ate some chocolate'," he said.

"Well there is a real sin. It's not putting a pound on, it's actually child labour giving us cheap chocolate.

"That's the real sin, that's the blood on our teeth, that's [what] we have started there."

The chief executive of Confectionery Manufacturers of Australasia, Trish Hyde, agrees with World Vision that child labour is a serious issue that needs addressing.

But she has countered Mr Costello's claim that the chocolate industry is resisting improvement.

"Far from resisting pressure to clean up its work practices, there is significant work being undertaken by the industry," she said in a statement.

"On the ground programs that are raising awareness and changing communities are underway now. Industry is supporting education for cocoa farmers on safe, responsible labour practices to reduce children's exposure to potentially hazardous situations.

"This education through farmer field schools also helps farmers get better crop yields, which means farming families are better off."