A former politician's alleged sexual abuse of a young teen caused mental struggles for the boy as he tried to process what happened, a jury has heard.

Ex-state MP Rory Amon, 36, has pleaded not guilty to a swathe of criminal acts against a 13-year-old boy at a Sydney apartment complex in 2017.

The pair met up on a gay hook-up site before talking over Snapchat and arranging two meetings in a car park bathroom, where rape and other offences allegedly took place.

The now-22-year-old alleged victim returned to give evidence in front of a NSW Supreme Court jury on Monday.

Rory Amon had listed his age accurately as 27 on the dating site, his lawyer submitted. 

Under questioning by Amon's barrister Matthew Johnston SC, he said he would not have fully understood the first meet-up was for a sexual encounter.

'I was 13 - I didn't really have a script for this kind of experience,' he told the jury.

Having earlier expressed excitement to a friend at the prospect of a romantic relationship, the short sexual encounter with Amon was not what he expected.

'I was developmentally not ready to be having sex, let alone with somebody twice my age,' the alleged victim said.

He divulged what happened to a friend soon afterwards, the jury heard.

'I feel like I was exposed to all of this way too early and it's kind of hard to process,' he texted her over Instagram.

'My anxiety about it is that bad and I get stomach pains.'

Mr Johnston suggested his client had listed his age accurately as 27 on the dating site, not 17 as the complainant had testified.

'I am certain that that is not true,' the alleged victim replied.

The younger man admits lying about his age to sign up to the 18-and-over website.

He lied again about his age to Amon later, testifying he presented himself as 15 years old.

Amon denies allegations he knew the complainant was under 16.

'I suggest to you, you communicated to the accused you were 17 years old,' Mr Johnston said.

'I am certain that is not possible,' the alleged victim replied.

On Friday, jurors were told that Amon and the teen - who was in year 8 - met up twice in the apartment block's communal bathroom, where a number of sexual acts allegedly took place.

The pair are alleged to have kissed, touched and had sexual intercourse.

On the second occasion, Amon allegedly brought along a towel so they could lie down on the bathroom floor.

Amon admits meeting with the teen on one occasion but says he never returned for a second visit - an assertion contested by the alleged victim on Monday.

'My memory serves me right ... there were two incidents,' the now-22-year-old told jurors.

'The second occasion did not appear out of thin air.'

The complainant told his teacher, psychiatrist, parents and friends in 2017 about the alleged assaults.

He did not make a formal complaint to police until years later after Amon became a local councillor and the boy realised the politician's true identity.

The former MP 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.

The trial continues.

1800 RESPECT (1800 737 732)

National Sexual Abuse and Redress Support Service 1800 211 028