A rapid rise in the takeup of pills to prevent HIV infection in some parts of Australia has been accompanied by a steep drop in the numbers of men using condoms during sex with other men whether or not they are on the protective drugs, a major study has shown.

Pre-exposure prophylaxis, or PrEP, has been hailed as a game-changer in the Aids epidemic, but the Australian experience suggests the availability of once-a-day pills that reliably prevent transmission of the virus may play a part in complacency about the chances of becoming infected.

But experts say PrEP is not solely responsible. Condom use among gay and bisexual men has been declining for some time. One of the reasons will be the knowledge, thanks to research, that men who are on a cocktail of HIV drugs do not pass the virus on during sex.

However, the new study, published in the Lancet HIV journal, raises serious questions about the introduction of PrEP in developing countries with high levels of infection without a strong package of educational support to encourage condom use.

The research, which involves 17,000 gay and bisexual men in Victoria and New South Wales, shows that PrEP has been eagerly adopted. Between 2013 and 2017, the numbers of HIV-negative men taking the pills to protect themselves rose from 2% to 24%.

Over that same time period, the proportion of HIV-negative men on PrEP having casual anal sex without using a condom went up from 1% to 16%. But overall, consistent condom use dropped from 46% to 31%.

Of greatest concern is that the proportion of men without HIV who were not on PrEP having casual anal sex without using a condom went up from 30% to 39%. However, at the same time, new HIV diagnoses fell in the two states.

“Our findings suggest that the rapid uptake of PrEP disrupted condom use at a community level,” said Prof Martin Holt at the University of New South Wales, who led the research. “However, it’s too early to tell the long-term effects of increasing PrEP use ... declining condom use may impede its long-term population-level effectiveness.

“We need better monitoring and evaluation to assess the effect of PrEP on sexual practices at the community level in both PrEP and non-PrEP users. If sustaining condom use is important, it would be wise to implement community education campaigns to promote condom use as PrEP is being introduced.”

Dr Michael Brady, medical director at the Terrence Higgins Trust in the UK, said the drop in condom use was not likely to be just about PrEP. “Condomless sex has been going down in gay men for a decade,” he said. “Rates of gonorrhoea and syphilis are going up. That’s long before PrEP happened.”

PrEP, he said, was “dropping into a situation that is already complex, where behaviour is already changing”. It would be contributing to the behaviour change but not the sole cause, he said.

Even though condom use is dropping in Australia, “I would be surprised if the HIV rates are going up,” he said, pointing out that as more people are diagnosed and put on antiretroviral drugs to suppress the virus in their body, fewer people will be infectious.

Deborah Gold, chief executive of the National Aids Trust in the UK, said the paper showed a situation that was unique to Australia. “But what we can say is that where there is less cultural fear of HIV, one may expect to see less condom use,” she said. “Indeed, condom use had been declining for years in England before the advent of PrEP, because people know that HIV is no longer a death sentence and HIV-positive people on medication can’t pass the virus on.

“It is also worth remembering that condom use is still significantly higher in gay and bisexual men than in the heterosexual population; taking PrEP is an additional way of proactively taking responsibility for your own HIV risk.”

• This article was amended on 8 June 2018 to specify that some of the cited figures referred to sex without a condom.