A source told The Daily Telegraph the duo — 40 and 24 respectively — were "full-on snogging" at the Australia's Got Talent (AGT) wrap party in Melbourne recently.
"They were spending a lot of time together on the day and had been dancing at the party and, well, one thing led to another and before you know it they were full-on snogging," said the source.
But Dannii's rep said there's "no truth" to the report.
"This story is completely fabricated. Dannii did not kiss or get caught in a 'clinch' with Johnny Ruffo at the AGT wrap party. There is absolutely no truth in this whatsoever," said the singer's rep.
Johnny came third in the 2011 X-Factor series and has since established himself as quite a stud-muffin.
He claimed the runner-up crown in Cleo's "Bachelor of the Year" competition and won the 2012 season of Celebrity Dancing With the Stars.
Meanwhile, Dannii's ex Kris Smith is said to be engaging in a reality TV romp of his own.
The 33-year-old footballer-turned-model is reportedly dating former Take Me Out — a UK dating show — contestant and glamour model Sara Beverley Jones.
