A 16-year-old boy allegedly took “calculated steps” to lure “a vulnerable young child” into a toilet block in Adelaide’s southern suburbs before sexually assaulting him, a court has heard.

The teenager was arrested in February this year and charged with three counts of rape, one count of taking a person by force and one count of aggravated assault over the alleged incident.

A prosecutor told the Adelaide Youth Court on Monday that the alleged incident occurred in a toilet block at a park in Flagstaff Hill.

“Those allegations, Your Honour, show a set of circumstances, set of behaviour that is deeply troubling,” the prosecutor said.

“There is an element of calculation, in my respectful submission, and pre-planning that ultimately leads to a violent assault.

“Those are calculated steps to ensure that this complainant ends up in a vulnerable setting.

“A set of circumstances … that are thought out to arrange for that complainant to be in that area at that time and then to facilitate an assault.”

A sign that says "Youth Court" is affixed to a brick building.

The magistrate refused the prosecution’s application to have the teen tried as an adult. (ABC News: Che Chorley)

The court also heard that the accused teenager was later “seen on CCTV running from the location”.

The prosecutor said the 16-year-old boy should face trial in an adult court because of the seriousness of the charges.

“[The alleged offending is on the] most serious end of the scale of criminality for offences of this type,” the prosecutor said.

“[They are] offences … committed on a stranger, a vulnerable young child … in a particularly vulnerable setting.”

The Adelaide Youth Court can order that a youth be tried in a higher court under Section 17 of the Youth Court Act if it finds the “gravity” of the alleged crime warrants a harsher penalty.

Magistrate Michael Jandy refused the application and adjourned the hearing.

The teenager is expected to face the Adelaide Youth Court again next month.