SYDNEY, Aug 14 AAP - When a Sydney nurse got a call from his lover's ex-boyfriend allegedly asking whether the man had told him he had HIV, he tried to put it "out of his head".

Weeks later, he was diagnosed with the disease.

The nurse, who cannot be named, cried as he told the Downing Centre District Court on Wednesday how he allegedly found out that his former lover - known only as MA - had HIV.

He said he received a call from MA's ex-boyfriend while the pair was holidaying in Vietnam in 2004.

"Don't be an idiot," the nurse said the ex-boyfriend had told him.

"I'm trying to save you, has the c*** told you?"

The nurse said "I put it out of my head", when MA allegedly claimed "I would never do that to you".

The conversation continued to niggle at the nurse for a number of weeks before he underwent testing.

In August 2004, he was diagnosed with HIV.

MA has pleaded not guilty to one count of maliciously causing a person to contract a grievous bodily disease and one count of maliciously inflicting grievous bodily harm.

The court was told the pair had met on the website Gaydar in late 2003.

Both of them had selected "always" in the safe sex category, the nurse told the court.

He said they did not use protection when first having sex because MA allegedly stated he didn't have HIV.

"He (MA) said ... I wouldn't ever do that to anyone," the nurse alleged.

In his opening address, defence barrister Michael Coroneos told the jury that "based solely on expert evidence you cannot be satisfied beyond reasonable doubt that MA was the one who infected" the nurse.

Mr Coroneos said the nurse could have contracted the disease from work.

He also said their relationship was "not consistent" with a monogamous couple.

Despite the nurse testing positive for HIV, the pair's relationship continued until December 2006.

Two weeks after the break-up, the nurse went to make a statement to police, Mr Coroneos said.

The trial before Judge Mark Marien continues.