The comedienne, who played Sharon in the Australian comedy series, joined diet programme Jenny Craig last December after suffering from weight-related medical issues.
The TV star told Australian Women's Weekly: "I feel like I've climbed Mount Everest. I can honestly say, I can't remember a time in my life when I felt better."
Szubanski revealed that she was suffering from sleep apnoea, a bad back, hip and knee pain and a recurring lung infection before she lost 25kg in weight.
The 48-year-old continued: "I just feel so well and have so much energy, not just for exercise, but for life. That niggling worry that used to be there sabotaging everything I did, that anxiety is gone."
Szubanski added that she has not been at her current weight since the 1980s.