Boyfriend Jay Lyon told Insider the pair had recorded a song together but wouldn't say what it was called.
"We have written a song together and we have recorded something together, but there are no plans for a release," said Lyon, 23, of New York-based Australian band Tamarama.
"It is kind of our little thing. She is a good little singer. She's got a sweet little voice."
Lyon, whose real name is Brent Tuhtan, was also a model. "Really, the focus is on music at the moment," he said.
Tamarama has made a strong impression in the US since forming four years back.The band recently signed with Brooklyn Beats, a division of Warner Music, and is working on a debut album, as well as a national tour of the US, playing mainly college shows. It has also got support gigs booked for Ryan Cabrera and Amy Winehouse's upcoming New York shows.
And fashion designer Tommy Hilfiger had the boys play at one of his recent parties, before guests including actors Liv Tyler and Michelle Rodriguez.
"We are getting taken care of now by the big boys, which is going to be good," said Lyon.