Trevor Dale pleaded guilty to one count of buggery and one count of indecent assault of a male, with the offences occurring in Sydney in late 1979 and early 1980 when Dale was aged 20.

His victim was just 10 at the time.

Two other counts of indecent assault of a male will also be taken into account during his sentencing.

Court papers said the first offence happened on December 29, 1979 when Dale had gone to the victim's home with his parents.

While playing in the backyard with the boy Dale fondled him and later forced him to perform oral sex on him, which "caused the youth to dry retch", police said.

Court papers said the victim was told not to tell his parents or he "would get into trouble".

A second offence happened about a week later when the boy was again forced to perform oral sex on Dale.

The assaults escalated on the third occasion to include anal rape.

Police said Dale covered the boy's mouth to stifle the his screams of pain.

The victim reported the allegations to police in August 2017.

The court heard police secretly recorded telephone calls between the victim and Dale in April last year where Dale allegedly made admissions regarding the incidents.

In one conversation Dale stated he was hoping the contact wasn' ringing to say, "Hi how are you? It's been 30 years since you molested me."

Over two phone calls they spoke about the sexual assault with Dale describing it "as a bit of fun" and how the victim's father "would have kicked his arse if he had found out".

"But you were just a child, you had no idea about your sexuality," Dale allegedly said during the conversation.

Following investigations, Dale was arrested at his Nowra home on April 11, 2018.

Dale initially denied having penetrating with the boy but pleaded guilty before Judge Siobhan Herbert in Sydney's Downing Centre District Court on Tuesday, August 20.

He will be sentenced on November 8.

Read more: Crime/Court