After coming up an escalator into the departure area, William and Kate waved to about 200 travellers and members of the public who had gathered on a viewing platform before they were whisked away to a private lounge.
Japanese national Yoko Asano said she'd just arrived from the Solomon Islands and had seen the royal couple there during their recent visit.
"I'm kind of following them," she joked.
"I didn't really expect to see them here but it was good because it was less crowded here.
"I could get a good look at them."
Kate wore a pale grey, floral-patterned, summer dress and beige patent heels while a tanned Prince William was equally casual in a jacket and white shirt, which was unbuttoned at the neck.
The Duchess used the stopover to change into a casual, loose white sweater and a colourful scarf, and dark, fitted pants.
A Brisbane resident, who would not give her name, said the Duchess looked beautiful.
"It's such a shame about the photos in France," she told AAP.
"People should just leave them alone - they're only human."
A judge this week banned French celebrity magazine Closer from any further publication or resale of the topless pictures of Kate - snapped while the couple were holidaying at a private estate in southern France.
The court ordered the magazine to hand over all copies to representatives of the royals within 24 hours.
The grainy shots are already widely available on the internet and have been printed in Ireland's Saturday Daily Star newspaper and Italy's Chi magazine on Monday.
"They welcome the injunction that's been granted, they always believed the law was broken and that they were entitled to their privacy," a royal source said on Wednesday.
The young royals are also awaiting a decision from France's criminal prosecutors about whether they will bring criminal charges against the photographer who took the pictures.
"From their point of view they were angry when they heard about the photos but once they took the decision to pursue a civil and then criminal action, they were able to put the matter behind them," the royal source said.
"Since they set the ball rolling it's now down to the law to take it's course."
It was announced on Tuesday that Prince Charles and wife Camilla will head Down Under in November when they will also visit Papua New Guinea and New Zealand.
- AAP
