Forster said Hartley was extremely remorseful of his actions and had been seeking appropriate help since police intervention last year.
A summary said Hartley found the girl's cellphone number through social media and texted her: "Hey chick xx."
Crown Prosecutor Fiona Cleary said despite knowing the victim was underage, Hartley continued with a "barrage of text messages", some of which were explicit.
Cleary said there was clear communication from the victim that she was 15 and not 16 until 2018.
She said Hartley continued to "harass" the victim despite knowing her personal circumstances and his actions represented "high end grooming behaviour".
Hartley sent texts to the victim insisting that they "meet up".
The Crown said the victim felt "pressured" and "blackmailed" by Hartley, who remained persistent in asking her to meet for sexual contact.
On December 12 she replied saying "well if I don't meet you you're going to f***** tell my mum."
On the same day, they met in farmland a short distance from the victim's address where Hartley rubbed the victim's breast and crotch area over clothing.
Police were contacted after the girl blocked Hartley's cellphone number on December 24.
Judge Bridget Mackintosh took into account that it was Hartley's first sexual offence as well as that he was going through a hugely "stressful" time in his life.
"Rather than seek help, you resorted to this situation," she said.
Hartley admitted his actions and was receptive in receiving help from the appropriate authorities, in which he was making progress, he said.
He always turned up on time for his counselling sessions, understood the impact he had on the family and said he was remorseful.
