Fiona shared photos of her postpartum body - one with her tights pulled over her stomach and one baring it all to remind women "how different bodies can look with just a slight adjustment of clothing".
"To all the mammas out there struggling with self-love and body confidence, I hear you! It’s not easy," she wrote to her 105,000 Instagram followers.
"On the days I struggle with my body Hayley reminds me that my tummy, or ‘Hunter’s Pouch’ as she affectionally nicknamed it, carried our beautiful son for 9 months keeping him warm and safe until he arrived in our arms."
The Biggest Loser star said her fiance's reassurance was the "perfect reminder" to be kinder to herself.
"The female body is incredible! So here it is in all her glory my pouch. It’s a reminder to never compare yourself to anyone else. Just live your own life and be happy!" she wrote.
Last month Fiona opened up to New Idea about struggling with not being able to work out due to her complicated pregnancy.
"For me, going for that period without moving my body, I did gain some weight and just started to not feel like myself," she said.
"My tummy went from holding him to just being soft and nothing there. But Hayley has been amazing. She’s just bolstered me the whole way."
As the new global ambassador for The Healthy Mummy, Fiona’s on a mission to get her health and fitness on track.
"I just want to be my best self for my family," she said.
"If training and eating well was about numbers on the scale, I would be tiny and I’ve never been tiny. For me, living a healthy, happy life is what I’m passionate about."
As part of her ambassadorship, Fiona recently undertook The Healthy Mummy’s 28 Day Weight Loss Challenge.
"It’s really versatile. There’s vegetarian meal plans, there’s keto – you can tailor it to what you’d like," she said.
"There’s also every type of training, including postpartum safe exercises and meditation."
Fiona and Hayley, a Nine sports journalist, got engaged in April 2019 after eight months of dating.