The couple, who started dating a few months back, also visited Wild Life Sydney Zoo to see koala, reported Daily Mail online.
"They never requested anything and just enjoyed a typical experience. It was just the two of them and while they did not want any photos taken, they were very pleasant and friendly," a source as said.
It was Westlife star Brian McFadden and model Ruby Rosem, who revealed the presence of those lovebirds in Australia.
"Having dinner with Mila Kunis. As you do," the 32-year-old singer tweeted.
"I am having dinner with Mila Kunis and Ashton Kutcher. I will have to see her in London!" Ruby also tweeted.
Later, Ruby posted a photo on Twitter with a caption, "dinner last night.. Everyone's hall pass... And Ashton."
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It is still unknown how long the duo will stay in Sydney.
Kutcher, 34, is currently taking a break from filming 'Two and a Half Men', while Kunis will have some movie projects in the near future.