A victim of child sexual assault was shocked to take a call at her work one day in 2013 from former Australian Olympics fencing instructor Michael O'Brien.

She immediately hung up and left work in tears - for the man, who by coincidence had made the call, was the same predator who had abused her decades earlier.

On her way home she reported the assault to police whose investigation led to O'Brien, now 90, pleading guilty yesterday to a count of gross indecency with a girl under 16.

O'Brien, who once coached Australian Olympic and Commonwealth Games teams, admitted the assault on the girl in the 1970s.

Michael O'Brien had started training the young girl in fencing when she was seven. (O'Brien Academy of Fencing)

The County Court of Victoria heard the victim took fencing lessons with O'Brien at the German Club in Richmond when he was head coach.

She later began lessons with him in his backyard shed in Balwyn, after he suggested to her parents that she train more.

Prosecutor Carolyn Burnside said the girl trained every weekend at O'Brien's house between 1977 and 1980, when she was aged from seven to 10 years old.

After one training lesson, O'Brien gave her a book on anatomy to read, deliberately opening it on a page showing female and male genitalia.

Then he told her he was going to have a shower.

Shortly after, he called her and she walked towards his room to find him standing naked.

Ms Burnside said the victim apologised and returned to the living room, but the accused called out to her a number of times, reassuring her he was wearing underwear.

He then requested a massage, saying he had a sore back.

The victim resisted but eventually agreed and O'Brien asked that she also massage his chest.

The court heard O'Brien asked the girl to touch him indecently and he also touched her.

"She recalls being extremely scared," Ms Burnside told the court.

The girl returned to the living room and waited for her mother to pick her up.

She didn't return to fencing after the incident.

The victim first complained to family and friends when she was about 15.

She later moved interstate and began working for a company where, by sheer coincidence, the accused called and she answered.

The accused was interviewed by police in 2014 and admitted he knew the victim and taught her fencing at the German Club and at his house. He denied the offending but admitted the girl saw him naked, having walked in on him after a shower.

O'Brien was charged in 2015 and was due to face trial, but entered a guilty plea yesterday.

Ms Burnside said it was an important day for the complainant, given she never wanted to endure a trial.

O'Brien, who is on bail, will face a further pre-sentence hearing on Wednesday.

According to the website for the O'Brien Academy of Fencing, which he founded, O'Brien is one of a select few in Australia to have qualified as a professional fencing master.