(CNN) Rates of new gonorrhea diagnoses among Australians rose 63% in just five years, reveals a new report on the nation's sexual health.

Australian health experts highlighted the alarming rise -- from 62 to 101 infections per 100,000 people -- and the need for people to be more aware of the infection as the reasons behind the trend are not yet fully understood.

There were more than 23,800 new cases of gonorrhea diagnosed in 2016, and about 75% of them were among men, according to the Annual Surveillance Report on HIV, viral hepatitis and sexually transmissible infections in Australia, published Monday.

In males, rates of gonorrhea infection were highest in 20- to 29-year-olds last year, while in women they were highest in 15- to 24-year-olds. But older age groups also saw increased numbers.

Infection rates also rose significantly in major cities, which saw a 99% increase between 2012 and 2016, while remote areas saw a decline, the report shows. For example, among women, there was a 126% increase in major cities compared with a 43% rise among all Australian women.

"The increase was most pronounced in the past two years," said Rebecca Guy, program head with the Surveillance Evaluation and Research Program in The Kirby Institute for Infection and Immunity in Society at the University of New South Wales, who compiled the report.

The reason behind the rise is not fully understood, Guy said. "What we do know is that more testing doesn't explain this trend," she said, adding that factors such as changes in sexual behavior, differences in screening and treatment practices, or differences in a particular strain in urban centers could be possible explanations.

"The data really highlight the need for greater awareness among clinicians, young people and adults about gonorrhea," said Guy, stressing that the condition is largely asymptomatic: Eighty percent of women and 50% of men won't have symptoms. "Therefore, regular testing is important."

In the United States, rates of gonorrhea infection have also been rising since 2009, according to the US Centers for Disease Control and Prevention, and are higher than rates in Australia, though the rise in recent years has been more modest.

"Australia is seeing some of the same trends of increasing (sexually transmitted disease) rates that we are in the United States," said David Harvey, executive director of the National Coalition of STD Directors, who was not involved in the report. "There are many factors at play here, but we have to acknowledge that as HIV becomes a more treatable and medically preventable disease, and condom use may be decreasing for some, we must continue to ensure that STD testing and treatment is prioritized, particularly in populations at increased risk for all STDs."

"Left untreated, gonorrhea is associated with serious long-term adverse health effects, including pelvic inflammatory disease, ectopic pregnancy and infertility.," said Matthew Chico, assistant professor of public health at the London School of Hygiene & Tropical Medicine in response to the findings.

"Particularly concerning is that we are rapidly facing the day when gonorrhea is no longer treatable due to antimicrobial resistance," he said.

The infections seen in Australia had reduced susceptibility to drugs, meaning they are moving toward resistance, Guy said. But she doesn't believe this is causing the rise in infections.

Highs, lows and some stability

Chlamydia infections also remained high in Australia, with more than 71,000 diagnosed in 2016, 75% of them among 15- to 29-year-olds, the report states.

Syphilis infection rates more than doubled over the past five years, increasing 107%. However, numbers are much lower than with the other infections, with more than 3,360 infections in 2016. Again, the greater number of infections was among men, particularly gay and bisexual men, Guy said.

She added that this rise is related to an increase in people getting tested for the infection.

But the report found some progress in terms of sexual health. HIV infections remained stable in the overall population between 2012 and 2016, with just over 1,000 new diagnoses in 2016.

"We're seeing increased uptake of HIV testing, particularly among gay and bisexual men, who are the population most affected by HIV in Australia," Guy said.

However, Chico highlighted that gonorrhea can increase the spread of HIV by aiding its transmission. "HIV has remained stable over the last five years," he said. "However, gonorrhea facilitates the transmission of HIV. Thus, the rise in gonorrhea in Australia may contribute to new HIV cases in the near-term."

Indigenous extremes

Gonorrhea rates fell among the Australian indigenous populations between 2012 and 2016, by 17%, but experts highlighted the fact that the numbers themselves remain disproportionately high: 6.9 times higher than non-indigenous populations in 2016, the report states.

Almost a third of infections were among 15- to 19-year-olds.

"Gonorrhea has been a significant issue in the population for a very long period of time," said James Ward, head of the Infectious Diseases Research Program-Aboriginal Health at the South Australian Health and Medical Research Institute. "We've got quite a bit of work to do."

HIV infection rates rose by 33% among aboriginal and Torres Strait populations -- indigenous to Australia -- with infection rates about twice those of non-indigenous populations. Routes of infections, such as through injection drug use or heterosexual transmission, were also more wide-ranging among indigenous populations.

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Ward believes less uptake of treatment and services plays an important role.

"There's a real divergence occurring," he said, highlighting the different profile in terms of how infections are occurring. There are large efforts among men who have sex with men in terms of prevention efforts in Australia, he said, which account for 74% of infections among non-indigenous populations.

"We've got a long way to go," he said.