They had argued the judge who sentenced Harry Smith to 18 months behind bars made a mistake in describing his 15-year-old victim as a "willing participant".

Smith was ordered to serve nine months before becoming eligible for parole.

The then-20-year-old met the teen at an oval in Sale, in eastern Victoria in September 2020.

They walked together before he kissed her and sexually penetrated her multiple times.

The girl had travelled there to meet Smith and on a walk around town she told him "no" when he asked if he could kiss her.

The girl's mother and police had already told Smith it would be illegal to have a sexual relationship with her because of her age, and he had been sent a letter repeating the warning by the Department of Health and Human Services.

When interviewed by police, Smith told officers "little girls can go out, f*** older guys and they get off scot free".

Three appeal judges on Friday rejected the appeal, finding the sentencing judge had not considered the girl's "willingness" to engage in sexual activity a mitigating factor.

"It was open to the sentencing judge to take into account the context in which the offending took place ... including the complainant was willing to spend time with (Smith) and had travelled from Moe to meet with him," Justice Karin Emerton said in a reason backed by two other Victorian Court of Appeal judges.

She highlighted the difference between the girl's willingness to spend time with Smith, as opposed to her willingness to engage in sexual activity with him.

Justice Emerton also noted that consent did not make an offence of sexual penetration of a child under 16 less serious.

The judges also rejected an appeal by prosecutors on the basis the sentence was manifestly inadequate.

Justice Emerton's remarks note Smith is "no ordinary sexual offender".

"He is a young person with pronounced vulnerabilities, disabilities and inabilities including an inability to understand and cope with the complexities of sexual interaction," she said.

While the sentence was "remarkably low" it was open to the judge, she said.

Appeals against sentences for assaulting police officers were also unsuccessful.

Australian Associated Press