The rate of diagnosis was highest in people aged between 20 and 39 and almost all were gay men. Nearly 600 of them were in NSW. The Kirby Institute head of surveillance and evaluation, David Wilson, said that in the 1990s the disease was almost non-existent. "Most people do consider syphilis to be a rare and isolated disease, not really of relevance today, and even 15 years ago that was true," Associate Professor Wilson said. "But we've seen a real resurgence, particularly among gay men and HIV positive gay men."

Historical data indicates that syphilis rates peaked in 1920 but there was a dramatic decrease after the advent of antibiotics. "What we're seeing now is the highest rates in recorded history," Associate Professor Wilson said. "Before penicillin, syphilis would have been higher than it is now." The report - HIV, Viral Hepatitis and Sexually Transmissable Infections in Australia - also indicated an increase in notifications of gonorrhoea with 14,947 notifications in 2013 compared with 13,842 in 2012. There has been an 80 per cent increase in gonorrhoea notifications in the past five years.

The most prevalent sexually transmitted infection was chlamydia, with 82,537 new diagnoses in 2013. NSW Health data also released this week indicates that chlamydia and gonorrhoea notifications rose sharply in the first quarter of this year, with 1211 cases of gonorrhoea and 5963 cases of chlamydia. Communicable Diseases Branch acting director Christine Selvey said most of that increase could be attributed to more testing. But this was not the case for syphilis, where the reason for the increase was a mystery. "That's the million-dollar question that everybody is asking themselves," Dr Selvey said.

"We don't really understand why it is." The Kirby Institute report also found infection rates for hepatitis C remained stable but more Australians were dying from it. An estimated 220 people in NSW died after contracting the virus, more than double the number of fatalities recorded in 2003. Associate Professor Wilson said he expected the hepatitis C death rate to continue to rise. "There are a lot of people who are progressing in their disease," he said. "It's going to get a lot worse in years to come and we do not really have the strategies or the will to make a big difference about it."

Dr Wilson said ensuring people with hepatitis C got diagnosed, and getting those with the virus to return to their clinician for treatment, would help address the death rate. UNSW social health researcher John de Wit said young people and gay men were at risk of getting hepatitis C because they were more likely to inject drugs or be exposed to intravenousdrug use. There was a stigma around injecting drugs that could prove a barrier to people seeking treatment, he said. A newly developed medication for hepatitis C has a 95 per cent cure rate and is likely to become available in Australia within the next couple of years.