The post was soon flooded with messages of support from Jess' industry pals.
Sports presenter Erin Molan shared some love through heart emojis, writing: "❤️❤️❤️❤️❤️❤️."
British TV presenter Trisha Goddard also sent her well wishes, penning: "Awww! Happy birthday to your darling girl!❤️🎉🎂."
Australian swimmer Stephanie Rice was equally touched by the stunning photos, adding: "Two gorgeous girls 😍."
Jess also took to her story to share a photo of both of her daughters in a car on Allegra's special day.
With the birthday girl in the passenger seat and her youngest, Giselle, 12, in the back with some large decorations, Jess captioned the photo: "Giselle squished with balloons!"
"Allegra thrilled to be turning 15 today!" she added, poking fun at the teenager's straight face.
Jess shares two daughters with her husband Peter Overton, with whom she recently celebrated her 18th wedding anniversary.
When it comes to her children, Jess wants to emphasise the importance of feeling confident as they grow up, and not getting bogged down in aesthetics.
“I talk to them about being strong and powerful and having strong and powerful bodies," Jess told our sister site, WHO. "I’ve never been on a diet, I never will be on a diet, it’s not in our vocabulary, so I’m very aware of that and I kind of celebrate my body.
“As my daughter says: ‘Mum, your bottom’s bigger than Kim Kardashian’ and I’ll say, ‘Well, I’m proud of it! My body gave birth to you and if I’ve got a bigger bottom since I’ve had you that’s got to be a good thing!'”