A former politician accused of sexually abusing a young teen in a car park bathroom has said he did not know the boy was 13 years old when they met.

Rory Amon talked to the alleged victim on a gay dating app and then over Snapchat before meeting at the boy's family home on Sydney's northern beaches.

The former state Liberal MP has pleaded not guilty to various criminal sexual acts against the teenage boy in 2017 and faced a NSW Supreme Court jury trial on Friday.

Then aged in his mid-20s, Amon initially spoke with the boy on a gay hook-up website after the teen lied about his age to sign up for a free trial, crown prosecutor Meaghan Fleeton told jurors.

Amon and the boy, who was in Year 8 at high school, did not use their real names.

They allegedly met on two occasions in June and July 2017 at the teen's apartment where he lived with his parents.

The sexual activity took place in a communal bathroom connected to the car park, Ms Fleeton said.

At the first meeting, Amon is accused of placing the boy's hand on his genitals and then directing the boy to have sexual intercourse with him.

On the second occasion, he brought along a towel which was placed onto the floor.

Further alleged sexual assaults took place while the pair were standing and then while they lay down on the towel.

Amon attempted to have sexual intercourse with the teen before telling him to perform another sexual act.

On both occasions, the former councillor left first.

The boy waited a few minutes alone before going upstairs to his home, Ms Fleeton told the jury.

The complainant - who cannot be legally named - then blocked Amon on Snapchat.

He told his teacher, psychiatrist, parents and friends in 2017 about what allegedly happened.

But the teen did not make a formal complaint to police until years later after Amon became a local councillor and the boy realised his real identity.

Amon was arrested and charged in August 2024.

Now aged 36, the former politician has pleaded not guilty to 10 charges including five counts of having sexual intercourse with a child and two counts of attempted sexual intercourse with a child.

He also denies indecent assault charges.

Amon admits meeting the teenager on a gay dating app but it may have been different to the one the Crown alleges was used, defence barrister Matthew Johnston SC told jurors.

The former MP also admitted talking with the teen on Snapchat before going to his home.

But Amon denied he knew the boy was 13-years-old as anyone who signed up for the dating website had to be aged over 18.

"At no point in time did (the alleged victim) provide Mr Amon with his real age," Mr Johnston told the jury.

Instead, the 36-year-old had an honest and reasonable belief that the complainant was at least 16-years-old, the jury heard.

"That is an innocent state of mind," Mr Johnston said.

The alleged victim is expected to tell the court he said he was at least 15-years-old.

Amon also disputed meeting up with the teen twice, saying he only went to the apartment block on one occasion, Mr Johnston said.

The trial continues.