Although she hoped to give birth naturally, she said her mantra was to "go through all means necessary so baby feels nothing", and when doctors recommended the surgery she immediately signed the forms.
"Within 20 minutes I had my baby in my arms," she wrote.
The Queensland-based personal trainer and founder of HIIT Australia, maintained toned muscles and a lean frame throughout her pregnancy - despite delivering a seven-pound baby.
The former Miss Universe contestant, who is 186 centimetres tall, only put away her boxing gloves and runners seven days before going into labour.
She attracted criticism for her barely-there bump, attributing the tiny belly to "clean eating".
"Each woman carries different and this most certainly doesn't mean one is doing something wrong or not healthy," she said at the time.
This image, showing a comparison with a friend who was at the exact same stage of pregnancy, caused a stir online, due to the difference in size of the women's bellies.
After a mandatory six-week rest, she bounced back into her training regimen, regaining her pre-birth body within months.
Baby "Miah" has now become a regular on her Instagram feed, as Duncan shares the highs and lows of her parenting experience with her 572,000 followers.
While most pictures document cute smiles, learning to sit and bath time, Duncan also reveals what happens when a green frappé doesn't quite go to plan.
She told her followers she would love to have three more children, and says she will try for a vaginal delivery.
"So to all the women out there wearing a caesarean scar, I am VERY proud of what mine means and for the beautiful gift I received through mine," she wrote. "They are memories of the day we all became mummies."
