One of seven men charged with sexually assaulting a 12-year-old girl who was offered up for sex by her father has been sentenced to five years and three months in jail.

Troy Phillip Milbourne, 42, had pleaded guilty to three counts of sexual penetration of a child under 13 and four counts of indecent dealings with a child under 13.

The offences occurred during a single day sometime between January and November 2014.

The District Court heard Milbourne was living a lonely life on a farm in Gingin when he responded to an advertisement the father had posted on Craigslist looking for company.

Milbourne met up with the father and the victim at a dog beach before going back to their house where the offences took place.

During a trial of issues prior to sentencing, Milbourne testified that he believed the girl was of legal age because that was what the father had told him, and because she was more developed and taller than his ex-girlfriend.

"Not being with a woman for so long, I thought she was 17 or 18, she looked it," he said.

Milbourne told the court he felt "sick" when police told him the girl's age following his arrest.

"I lost my whole world that day, I was gutted," he said.

"I felt like the next Rolf Harris," he told the court.

Milbourne said if he had known her real age, he would have reported the father to police.

But District Court Judge Alan Troy did not accept that, and said the girl's appearance and development was that of a child.

He said he found it unlikely that Milbourne would not ask the girl about what she did for work or where she went to college if he truly believed she was older.

"He has had five or six sexual partners ... And has experience as to what a naked adult female looks like.

"I am positively satisfied when he committed the offences he knew she was under 16," Judge Troy said.

Milbourne had also told the court that he had no intention to have sex with the girl when he was invited to their house.

He testified he used their en suite shower because he had been swimming at the beach, and was shocked to find the girl had entered the shower naked after him which led to them having sexual intercourse in the adjoining bedroom.

But Judge Troy rejected that, saying he must have been told by the father that he could have sex with the girl as there is no other reasonable explanation as to why they would have sex while the father was at home.

"It must have been obvious to you that she was being used as a sexual object by her father," he said.

Milbourne will be eligible for parole and his sentence will be backdated to June last year.

Seven men, including the girl's father, were charged last year with more than 500 offences against the girl after the paedophile ring was uncovered by police.

The father was jailed for 22-and-a-half years for sexual abusing his daughter and offering her to men for sex while he watched or participated.

He cannot be named to protect the identity of his daughter, who is now aged 13.