The victim was just 15 when she had sex with a man she thought was 17, but was actually four years her senior.

Zac Cashill, 22, pleaded guilty in Warrnambool County Court on Tuesday to sexual penetration of a child under 16 and sexual assault of a child under 16.

The first charge has a maximum penalty of 15 years' jail and a standard penalty of six.

The court heard the man was 19 when he started messaging the victim on social media and told her he was 17.

The pair met up at the offender's home where he touched the victim and had sex with her.

When the girl didn't return home, police were called and attending officers discovered the pair in bed about 4am.

When advised of the man's age, the victim said she felt "really upset" that he'd lied but denied any sexual offending.

She made a formal police report the following month.

In a victim impact statement, the victim said she now hated the body she walked in and the person she saw in the mirror.

She said her depression and anxiety was exacerbated following the offending and that she had lost a part of herself.

"He took away the respect I had for myself and my body," the victim said.

"He made me believe something that wasn't even true."

Barrister Julia Kretzenbacher, representing Cashill, said her client admitted lying about his age was a "stupid thing to do".

"In his words he said 'it's the worst thing that I did'. It is something that he has since reflected upon," she said.

She said the man had an intellectual disability and there were limitations in understanding the consequences of his actions.

She said Cashill had no criminal history, no subsequent offending and good prospects for rehabiltation.

Ms Kretzenbacher urged the court to consider sentencing the man to a community correction order with a disability justice plan.

But crown prosecutor Robyn Harper said the offending was serious and only a term of imprisonment could meet the necessary sentencing purposes.

Judge Michael Cahill said a jail sentence usually followed the serious offending Cashill had pleaded guilty to.

But he said he understood the defence submissions that the man's case was not a usual one.

He ordered the man be assessed for a correction order.

The judge said that did not indicate what the man would ultimately be sentenced to.

Cashill is expected to be sentenced on June 15.

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