A TRANSGENDER teenager, who sexually assaulted a 13-year-old boy over a period of almost 12 months, has been sentenced to 18 months in jail, wholly suspended.

Harley Leo Strachan, 19, who prefers to be known as Heidi, pleaded guilty in Hervey Bay District Court to four counts of indecent treatment of a child under the age of 16 and one count of deprivation of liberty.

The assaults dated from December 31, 2013 to December 14 last year.

Crown prosecutor Chris Cook said the assaults, which involved kissing and on two occasions oral sex, happened when the victim was aged 12 and 13.

The court heard the charges took place "in the context of a relationship" but did not appear to be a willing participant in one of the incidents, which resulted in the boy being held in a room.

"There are laws in place to protect young people from these acts which they have consented to themselves," Mr Cook said.

Mr Cook said the incidents, along with previous criminal history, suggested an escalation in his offending.

Defence barrister Patrick Wilson said Strachan had gone through significant difficulties throughout her life, including abuse suffered at an early age.

Mr Wilson said time Strachan had spent in pre-sentence custody, served in a men's facility, had been difficult, due to her gender status.

"She has been heavily victimised in that environment," he said.

Mr Wilson said Strachan reported being verbally or physically assaulted at least four or five times a week while in custody.

Judge Helen Bowskill sentenced Strachan to 18 months prison, to be wholly suspended for three years.