A Sydney man has become the second gay Australian to contract HIV despite taking medication that promises to prevent the spread of the disease.

Key points: Steve Spencer described receiving his diagnosis as being "a rollercoaster"

Steve Spencer described receiving his diagnosis as being "a rollercoaster" He says he holds no grudges against the PrEP medication and is a huge supporter

He says he holds no grudges against the PrEP medication and is a huge supporter PrEP is usually taken daily, but some users like Mr Spencer opt for an "on-demand" approach

Steve Spencer, 27, returned a positive result before Christmas following a regular sexual health test — a requirement for people taking pre-exposure prophylaxis (PrEP).

"It was a surprise and a shock," he said.

"I remember I was left sitting in the doctor's waiting room for 15 minutes and then came the diagnosis.

"It has been a bit of a rollercoaster, a bit of a wild ride, as anyone's diagnosis will be."

Most doctors recommend taking the blue pill daily for it to be around 99 per cent effective, but Mr Spencer was following the "on-demand approach" — which can be recommended to PrEP users who don't have regular sex.

A PBS listing reduced the cost of the HIV prevention drug PrEP. ( ABC Radio Darwin: Jesse Thompson )

Australian Society for Sexual Health Medicines Associate Professor Edwina Wright said that method involved taking two tablets before sex and another two in the following 48 hours.

"There's one excellent trial which shows reduced HIV transmission by 86 per cent," she said.

"We're still waiting on further studies to see whether it's just as effective as taking it daily."

Associate Professor Wright said not everyone was comfortable taking daily medication, due to the cost or toxicity, while others don't have enough sex to justify it.

'A little unlucky'

Mr Spencer started HIV treatment the day he was diagnosed and after six weeks his viral load was undetectable, meaning the virus is unable to be transmitted during sexual intercourse.

He said the last thing he wanted was for people to start doubting the effectiveness of PrEP.

"I can acknowledge why people would be fearful of this or have concerns for themselves or their loves ones," he said.

"I'm maybe a little unlucky, but that's how the cookie crumbles and I am not angry at all, I don't hold a grudge."

Mr Spencer was part of The Institute of Many's Mardi Gras float this year. ( Supplied: Nic Holas )

More than half a million people around the world take PrEP and health advocates hope the drug will help end the HIV epidemic by 2020.

"HIV rates are falling quite dramatically every year," said Darryl O'Donnell from the Australian Federation of AIDS organisations.

"PrEP is largely responsible for this and while Steve's case is unfortunate, there have only been a handful of men that have contracted the virus while taking the medication."

PrEP was put on the Pharmaceutical Benefits Scheme in April last year after clinical trials involving 14,000 gay and bi-sexual men across Australia.

That reduced the cost of a three-month supply from around $2,000 dollars to $120, with some PrEP users importing it cheaper from overseas.

Mr Spencer says he wants to help reduce the stigma attached to living with HIV. ( Supplied )

Mr Spencer has been a PrEP advocate since he first signed up for the NSW trial and remains supportive of the drug.

"Even though my case may sow a seed of doubt, I don't think it should at all," he said.

"There's no such thing as a perfect drug, as we've found out, but it is so near perfect that it is amazing."

The gay rights advocate chose to "come out" as HIV positive on a float as it made its way up Sydney's Oxford Street at this year's Mardi Gras parade.

He was greeted with cheers and hugs when he told the 80 participants — both those with HIV and their allies — he was revealing his status publicly.

"You could honestly not wipe the smile off my face and in fact you still can't," he said.

"I chose to come out this way to reduce the stigma attached to HIV — to show people that I am proud of who I am."