A “game-changer” HIV drug which cuts the risk of being infected by more than 90 per cent has led to a "striking reduction" in regular condom use among gay and bisexual men, Australian researchers have found.

Analysis of sexual health trends before and after daily pre-exposure prophylaxis (PrEP) drug, Truvada, became widely available in major Australian cities found even men not taking the treatment are less likely to use a condom.

PrEP’s roll out is driving noticeable declines in new HIV infections in many regions; one London clinic reported a 42 per cent drop, the first significant decrease in decades, even before it got wider NHS funding.

The drug kills off the virus before it can become established after transmission from an infected partner, but its effectiveness is lessened if it is not taken consistently and even daily use is not 100 per cent effective.

It also does not prevent the spread of other sexually transmitted infections and experts warn the declining condom use could lead to HIV and other diseases "rebounding" in unprotected men.

“PrEP has been heralded as a game-changer for HIV,” said Professor Martin Holt at the University of New South Wales, Sydney, who led the research published in the Lancet HIV journal.

“But declining condom use may impede its long-term population-level effectiveness.”

“Our findings suggest that the rapid uptake of PrEP disrupted condom use at a community level” he added, but said it is still too early to know the long-term impact on HIV being passed on.

The study surveyed men who have sex with men in Melbourne and Sydney on their HIV prevention practices and sexual encounters in the past five years.

It found, between 2013 and 2017, PrEP use by men without HIV rose from two per cent to 24 per cent – 783 of the 3,290 respondents.

In the same period, men on PrEP who had anal sex with a casual partner without a condom rose, from 1 per cent to 16 per cent. At the same time there was a "striking reduction" in regular condom use, falling from 46 per cent to 31 per cent, Professor Holt said.

Even men who were not protected by PrEP and who were HIV negative or untested reported they were more likely to have condom-less anal sex with a casual partner - rates rose from 30 per cent to 39 per cent.

While this period coincided with a fall in new HIV diagnoses suggesting PrEP’s benefits - and other testing and prevention schemes - are still outweighing the decreased condom use, but this cannot be taken for granted long-term.

“If individuals not taking PrEP feel safer, they might use condoms less often because they perceive that sex without a condom has become less risky as PrEP use by others increases,” Professor Hold added.