Prosecutors have successfully applied to escalate the case of Dylan Lloyd, who sexually assaulted a 12-year-old girl on a train in Adelaide's south, to the state's highest court.

Lloyd, 22, pleaded guilty in the Adelaide Magistrates Court in February to unlawful sexual intercourse with a person under the age of 14, and to indecent assault.

Prosecutors made the application to send the case to the Supreme Court under legislation relating to offenders incapable of controlling — or unwilling to control — their sexual instincts.

Lloyd abused the girl while they were travelling on the Seaford train line in January.

Police previously said Lloyd tried to follow the victim, who was on her own, before a third party called police.

The District Court heard on Friday that prosecutors applied for Lloyd to be dealt with in the Supreme Court, which was granted by Judge Anthony Allen.

Under the relevant legislation, the Supreme Court may order that offenders incapable of controlling or unwilling to control their sexual instincts be detained in custody until further notice.

"I am of the opinion that the defendant should be dealt with under the [legislation]," Judge Allen said.

"Instead of sentencing the defendant myself, I will remand the prisoner in custody to appear before the Supreme Court to be dealt with."

The legislation also states that before an order to further detain the prisoner can be made, the Supreme Court must direct at least two medical practitioners to examine the prisoner's mental condition, and report to the court on whether the person can control their sexual instincts.

Lloyd's defence lawyer Andrew Williams, who told the court his client had severe autism, also said Lloyd was charged "with breach of bond". 

"That breach will in due course be admitted," Lloyd's counsel told the court.

The breach relates to a bond made after an aggravated indecent assault charge Lloyd committed in August 2021 at Oaklands Park.

The matter returns to the Supreme Court in July.