The 43-year-old Offspring star, who plays Billie Proudman in the popular series, revealed the exciting news to The Daily Telegraph over the weekend.
Speaking to the publication, Kat said she didn’t mean to keep her second pregnancy a secret but it just panned out that way.
“It wasn’t a conscious thing. I haven’t been hiding her. I’ve just been pottering around at home,” she told the publication of her newborn daughter, who she affectionately calls Gigi.
Kat is already the proud mother to four-year-old son Archie, who she shares with her actor husband, David Whiteley.
The star is gearing up to return to work to film the sixth season of Offspring and her newborn will be right by her side.
“Gigi is a very pleasant surprise. I wouldn’t normally go back to work this early but Offspring is a family. Archie was on set with me when he was really little and I’m doing the same with Gigi,” Kat explained to The Daily Telegraph.
And Gigi will have a playmate in co-star Asher Keddie's one-year-old son, Valentino.
“We’ll have Asher’s little boy here too. It is a family-friendly atmosphere. It feels really right and really nice,” Kat explained.
In 2014, the actress said she had a great support network in her family, who help her with her son while she film.
"I am so lucky. Because David and I are both actors we have been able to make it work so that we can be home for him [Archie]. We haven’t had to use any sort of childcare. We have great family support," she told The Herald Sun.
For now, the star is relishing time with her new family of four before she enjoys her first night out since welcoming Gigi at the Logie Awards on May 8.