A former Sydney gynaecologist who was acquitted in two separate trials of all allegations of aggravated sexual assault against his patients will have all legal costs paid for.

Dr Richard Ian Reid, 78, was accused of more than 20 charges dating back to 1997 when he was performing surgery in St Luke's Hospital, and later privately practising in Sydney's Edgecliff and Double Bay.

Acting Judge Michael Allen said on Friday in the NSW District Court that had the prosecutors possessed all relevant facts it would not have been reasonable to proceed with both trials.

Acting Judge Allen granted Dr Reid a certificate for all legal costs incurred from both trials to be paid for by the Crown.

One of the complainants' accounts was "at best implausible and entirely unsupported by objective evidence," the judge said.

"(She was) not simply unimpressive, her version of events was inconsistent with other accounts she provided."

The retired specialist was arrested in September 2019 after the woman approached the police in November 2018, when she was 64.

The woman described a "'robotic and ritualised and unambiguously egregious sadistic sexual assault," over multiple occasions in his Edgecliff consulting rooms between 2010 and 2014.

Acting Judge Allen said one trial relied solely on this witness who lacked credibility from the outset, and whose evidence was inherently weak.

He pointed to a number of messages she had sent Dr Reid over the years, beginning with a Christmas card in 2011.

Following his surgical procedure on her in October 2012 she sent another note.

"The world would be a much better place if there were more people like you," she wrote.

"I'm lucky to have met you. Thank you so much Richard, I am getting better and will come for a check-up soon."

In 2015 she wrote that he helped her where all other specialists had failed, and undoubtedly would still be in agony had Dr Reid not treated her.

The judge said this was in complete contradiction to her evidence, where she described such excruciating pain during the examination she cried and asked him to stop, and claimed he had mutilated her.

She alleged that he tried to kiss her, gazed at her with a "wanting look", and at one point put his face between her breasts and said she smelled beautiful.

But this was dismissed by the judge.

He pointed to an insurance application for financial compensation she had made that claimed support from another gynaecologist.

But this gynaecologist in evidence said he told the witness during their final consultation that he did not support her claim, as her chronic health issues pre-dated Dr Reid's treatment.

The woman also omitted half of a medical report provided by Dr Reid that included his explanations for standing examinations, his diagnosis and treatment options.

The Crown's case had also relied on an expert witness to testify that Dr Reid's standing examinations of patients had no proper medical use.

But this witness lacked clinical experience and was contradicted by other expert testimonials that said there were cases where this method was legitimised.