WHETHER prostitution is legal or not has ''little or no impact on the size of the industry'' but it does affect the health of sex workers, Australian research shows.

A study of prostitution in Sydney, which has the most liberal approach; in Perth, where it is mostly outlawed; and Melbourne, which falls in between, has found a ''thriving'' sex industry in all three cities.

Legal status was found to affect only the health and support services available to sex workers and the hygiene and safety standards of brothels.

''In spite of the different legal climates, each city had a thriving brothel, escort, private [call girl] sex industry plus a small street-based industry,'' a co-author Basil Donovan, from the NSW and National Centre in HIV Epidemiology and Clinical Research at the University of NSW, said.

''On a per capita basis, the number of brothels was broadly comparable between the cities.