The model and Amazing Race winner, who recently broke up with former Neighbours actress Erin McNaught, was spotted having a "romantic" dinner date with Cox at Surry Hills restaurant Toko on Tuesday night.
One onlooker told The Daily Telegraph that the couple shared sushi and gazed at each other across the table, adding: "They were smiling a lot... It had to be a date."
Last month McNaught, who is on the books of Chic Model Management alongside Cox, was pictured kissing and flirting with an unnamed man in Bondi, but insisted that they were old friends.
McNaught said after her split from Joliffe: "I adore Nathan. We are still really close, but it was a matter of our lives conflicting. Our lives were slowly heading in different directions. So we decided to call it quits rather than go through that strain."
After being named Australia's Next Top Model a week ago, Cox said that she felt "incredible", but she was later forced to slam reports that she is too thin, saying: "I'm a very healthy girl and am always exercising. You do want to do the right thing and be a good role model for people, and not promote the idea that if you want to be a model you have to be skinny and look the right way.
"I am naturally thin, but I am a happy, healthy girl and that's what counts."
> Erin McNaught, Nathan Joliffe deny cheating rumours