The court was told the couple stopped what they were doing after they were approached by security, and Leeding got off Quinn who attempted to cover himself.
"The security officer left the area ... she then sat on his lap, facing away and again started moving up and down on his lap, appearing to again engage in sexual intercourse," Francis said.
The pair continued until they were again interrupted by a volunteer court staff member but continued once the staff member left the area.
"For a third time the couple were approached, again by security, and Leeding quickly stood up and held her arms out in an attempt to block the view of the security guard while Quinn attempted to cover himself up," Francis said.
The couple left the waiting area, which was full ahead of a busy court day, with members of the public less than 5m away.
The court was told they were easily identified by CCTV cameras, and on June 30 the couple was issued with notices to appear by police. On Friday the pair pleaded guilty to indecent act in any place to which the public is permitted access.
Solicitor Nathan Bouchier said Quinn's actions "weren't planned" and that the unemployed labourer simply "wasn't thinking" of how inappropriate their actions were.
Bouchier said his client was looking for full-time employment, after previously completing a construction traineeship, and had two job interviews next week.
Leeding's solicitor Ryan McCullough said the only explanation for the couple's behaviour as they had been overcome with the "exuberance of youth".
McCullough said the couple's "stupid and inappropriate mistake" was a hard learnt lesson in what was appropriate behaviour.
Magistrate Clare Kelly said she had never heard of such an incident happening at a courthouse, describing the offence as "outrageous behaviour".
"In a disrespectful manner you've taken into the public arena a very private act," Kelly said to the pair.
The couple was sentenced to perform 60 hours of community service each, with no convictions recorded.
