There were an estimated 260,000 new cases of chlamydia in 15 to 29-year-olds by the end of 2015, according to the Australian Annual Surveillance Report into sexually transmissible infections and blood-borne viruses released today.

That works out to roughly one in 20 young Australians. The University of NSW's Kirby Institute, which produces the report, estimates about three-quarters are undiagnosed and untreated.

"Chlamydia is curable, but most people, over 80 per cent, have no symptoms," said Associate Professor Rebecca Guy, from the institute. "We need to maintain the focus on more regular STI testing."

Reports of the infection in 15 to 19-year-olds decreased by 19 per cent over five years to 1271 cases per 100,000 of the population in 2015 but continued to rise in 20 to 29-year-olds, which researchers believe may be linked to an increase in testing.

Even so,  of all the young people who visited a GP in 2015 only 16 per cent were tested for chlamydia.

Chlamydia is treated with a single course of antibiotics but if left unchecked can lead to pelvic inflammatory disease and even infertility in women and testicle infections in men.

The report also reveals that despite HIV levels stabilising overall, new infections in Aboriginal and Torres Strait Islander populations have continued to rise, reaching the highest levels on record with 38 new cases in 2015.

The rate of diagnosis of HIV in those groups is more than double that in the non-Indigenous population and twice as high as it was five years ago.

"There are some underlying risk factors for the Aboriginal and Torres Straight Islander population that make it really susceptible to a rapidly increasing HIV epidemic if it was to take hold,"  Indigenous health researcher James Ward said.

"We've got a particularly young population where much more of our population live in remote and regional areas where access to HIV testing and treatment services is not as readily accessible as in the city."

Australian Federation of AIDS Organisations president Bridget Haire said funds had been stripped from the Anwernekenhe National HIV Alliance, which represents Aboriginal people living with HIV, in 2015.  "This is not helpful at a time when HIV rates are rising," she said.

In the non-Indigenous population about 75 per cent of HIV infections were through male-to-male sexual contact, but this accounted for only about half of the exposure to HIV in the Aboriginal and Torres Strait Islander communities, which had higher rates of transfer through heterosexual sex and injecting drugs.

A devastating syphilis outbreak has continued to spread through Indigenous communities in the north of Australia, with notifications of the disease increasing more than twofold over the past five years.

The outbreak, which in Queensland had reached 167 new cases and claimed three babies' lives within five months this year, prompted a $15 million commitment to attack the problem from the Queensland government in May.

Professor Ward said the outbreak started in north-west Queensland in 2001 and spread to the Northern Territory and Western Australia.  Infections are still rising despite the best efforts of a syphilis outbreak committee that meets monthly, he said.

"It does continue to elude us and there are lots of efforts. All of these notifications are epidemically linked to the north of Queensland and currently there's about 900 cases linked to this outbreak since 2011."

The surveillance report will be released at the Australasian Sexual Health Conference in Adelaide on Monday.