Fiona's fans and followers were quick to send messages of support.
"Sending you all so much love," one penned.
"I can’t imagine how hard this must be, sending so much love to you and your family," wrote another.
"Oh Fiona, this is heartbreaking take care lovely as you navigate this next stage," noted a third.
Back in January, Fiona shared a clip of her mum and Hayley dancing together in front of the TV over the Christmas break.
"My mum has Alzheimer’s she gets so much joy out of dancing and singing," she captioned her clips.
"We had not seen mum in over a year and my beautiful fiancée @hayley__willis spent a lot of time with her over Christmas making her laugh and smile which is getting harder to do as this disease takes hold. One of the many reasons I love her just a reminder to hold your loved ones tight."
Fiona and Hayley celebrated their first Mother's Day on Sunday since welcoming baby Hunter in March.
"Happy Mother’s Day to my beautiful partner in crime @hayley__willis," Fiona wrote in a loving post that featured a snap of Hayley gently kissing their son as Fiona looks on.
"Hunter and I would be lost without you! Being a Mum with you is better than I ever imagined. Every day is an exciting new adventure and I can’t wait for many more to come. Love your ball and chain."
The former Biggest Loser star and the Channel Nine sports presenter announced their engagement in April 2019 after seven months of dating.