Stewart won many new fans in her role as Billie Proudman on Offspring, as part of an ensemble cast led by Asher Keddie and including John Waters, Eddie Perfect, Deborah Mailman, Richard Davies, Garry McDonald, Lachy Hulme and Linda Cropper. One of the show's strengths is the dynamic relationship between the sisters Billie and Nina Proudman (played by Keddie). Both actresses have spoken of their admiration for each other, with Stewart saying "I've admired Asher ever since Love My Way. She is an Actress of extraordinary ability and depth. We have a lovely shorthand and support for each other on set, not unlike Nina and Billie. There's a trust there that allows real spontaneity and joy." Keddie spoke of Kat, saying "It's exhilarating to perform with her; I feel genuinely excited by it. That's a great feeling to have as an actor - when you really want to be working opposite someone and with someone. It doesn't get much more rewarding than that." Stewart admits to being fond of her character, Billie, despite some heartaches, "I’m really fond of her, though she’s really frustrating sometimes. Though she cheated on Mick [in season four], it’s my job to see the world through her eyes. You see what happened in the lead-up and it was the ultimate act of self-destruction. It broke my heart." Offspring aired for 5 seasons from 2010–14 before returning to TV screens for a sixth season in 2016. With production of the new series underway Stewart spoke to Colin Vickery of News Corp, "I really didn’t think it would happen", Stewart said. "It was the best experience I’ve had in a show. You form such deep relationships with the people and also the characters. I feel really confident we’ll be able to deliver something great. I don’t think we’re going to coast on some sort of legacy. We’re really aware of that and all want it to be as fantastic and awesome as it can be. To get to walk around in Billie’s shoes again is fantastic.” The seventh season of the show, including Stewart and other key cast members, screened in 2017. Writing in 'The Daily Telegraph', Colin Vickery reflected on season 7 with 'this year...Billie, played by Kat Stewart, has had to endure a heartbreaking split with husband Mick (Eddie Perfect). Billie’s rollercoaster life has showcased Stewart’s comedic and dramatic talents like never before. No wonder many critics believe she is the best actor on Australian television.' No decision has yet been made on whether the show will return for an 8th season.