Later, when the boyfriend told Fairburn she was only 15, he responded he was okay with that as long as it was kept discreet.
No further sexual contact took place, however, and the boyfriend was arrested a few months later, leading to the four men who had paid for sex.
The girl provided a victim impact statement to the court which was not read out but Crown prosecutor Richard Annandale said it detailed the offending's serious effects on her.
Defence lawyer Jarred Smith said his client didn't realise the victim was underage until afterwards, had no criminal history and was otherwise of upstanding character. He was also suffering personal stresses at the time.
Judge John McDonald said the Prostitution Reform Act 2003 had made it legal to pay for sex but also brought in strong measures, including a seven-year maximum jail term, so people under the age of 18 would not be exploited.
However, he was bound to some extent by sentences of home detention handed down by other judges to the other men involved.
"Consistency is a very important aspect of sentencing. No matter what judge a person comes before in the country, they should receive a similar sentence for similar offending," he said.
Judge McDonald took three years' jail as a starting point, reducing it for Fairburn's guilty plea, previous good character and difficulties in his personal life at the time to arrive at an end sentence of 10 months' home detention.
He also ordered Fairburn to pay the victim $1500 reparation for emotional harm.
The victim's former boyfriend has denied all 20 charges against him and is in custody awaiting trial.
Kaitaia man Marcus Barker was earlier sentenced to 12 months' home detention, Auckland church leader Michael Weitenberg to six months and Kawakawa's Owen Sigley to nine months. All were also required to pay reparation.
