The man, 58, claimed the pair formed a close relationship that became sexual after they began to confide in one another about difficult times in their lives.

He pleaded guilty to three counts of sexual intercourse with a minor and appeared before a sentencing hearing in Queanbeyan District Court on Thursday.

Prosecutors argued the man grew besotted with the girl and she became his "reason for living".

Police facts showed the former public servant met the victim when he was volunteer coach of a school sports team she played on.

He acted as a mentor to the troubled student after she told him she was struggling to cope.

The court heard the victim slept at the man's home several times when his family was present.

One time, the man picked the girl up and took her back to his house while his family was away.

The pair slept naked in the same bed, kissed and cuddled, but did not have sex.

The court heard the man visited a medical centre the next day to get a prescription for the erectile dysfunction drug Viagra.

They had sex that night, and on several other occasions.

Police were called after the man's estranged partner found inappropriate text messages between the pair on his phone and notified the school.

The man has since been removed from his coaching position.

Defence barrister Steven Whybrow said his client had spent two months in hospital for mental health problems since his arrest and couldn't remember some details of the pair's relationship.

From the witness box, the man said he was prompted to share incidents of abuse from his own childhood for the first time with the girl to help her overcome struggles in her own life.

He said he had "stuffed up" and didn't blame the girl for what happened.

"I was trying to be someone she could rely on who wasn't going to let her down," he said.

"She didn't need someone who was confiding their own problems."

The sexual side of the relationship "just happened very suddenly", he said.

He said the girl had initially asked him for sex but he told her it wouldn't be right.

He rejected a suggestion he offered to have sex with the girl to "make her feel better".

Under cross-examination by Crown prosecutor Kim Stanley, the man denied he was besotted with the girl and that helping her had become his focus in life.

He agreed with her assertion that taking Viagra showed "some premeditation".

The hearing will continue before Judge Chris Hoy next week.