"I think it's fab when I see people driven to eat and move better," she captioned a series of images.
"Whether that means setting yourself a seasonal fitness goal, feeling inspired by the sunshine to be more active, using a vacation or new year's resolutions as motivation, even setting your sites on toning up or losing weight to fit into a strappy dress."
However, she said there's a big "but".
"I want to set one thing straight. THERE IS NO SUCH THING AS A BIKINI BODY," she wrote.
"From stretch marks and loose skin, to people with chiselled abs or (what some might call) "bulky" muscles. From post-partum bodies to cellulite or TTOTM (that time of the month) bloating. From heavy, light, curvy, straight, jiggly and broad, to everything in between – all bodies are fit to rock a bikini."
The former Biggest Loser fitness coach shared several images of herself including one showing off her six-pack, with "loose skin" and after having just given birth.
She wanted to remind her more than 200,000 followers that the female body goes through changes and also to be mindful of the messages weight-loss companies share around this time of year.
"As summer hits, so does a lot of negative body image messaging. Lines like "get your bikini body" and "fit into your bathers this year" are thrown around recklessly, while only photos of the tightest tummies and perky everything else flood our feeds," she said.
"Because of this standard, for many real women (and men) summer can bring with it a lot of anxiety, doubt and pressure – anxiety about that hot day at the beach in bathers, doubt over wearing that singlet dress to Christmas, pressure to look a certain way."
Instead of focusing on the "perfect" body, Tiff set her fans a challenge, asking them to set "functional, strong and capable body" goals.
She said it is important to focus on finding confidence and nourishing your body as well as practising self love.
"Even if your goal is to lose weight (that's OK!), do it with self love."
"And remember: Wear that singlet, shine in that sundress, smile at yourself in the mirror, buy those bathers, take that selfie, share that photo – your body is incredible."
Tiff's post has been met with hundreds of comments from fellow women praising her words and honesty.
"Aww and this is why we love you so much Tiff," one follower wrote.
"Tiff, thanks for always keeping it real, always being supportive to EVERYONE," another fan added.
"Truth, realistic, an example, unafraid, down to earth and a motivator," added a third person, while a fourth wrote, "Thank you for this reminder … I desperately needed it".
After giving birth to her son Arnold and gaining 30kg during her pregnancy, Tiff
said she felt overwhelmed by being expected to "bounce back" within weeks of his arrival.
However, since then she instead decided to take on an approach to "bounce forward".
In one of a series of images she shared where she is wearing a pink bra, Tiff had previously said she was no less happy in it because her body was "bigger or different".
"And I'm five sizes bigger than my pre-Arnold body – but in this moment I don't care. I couldn't be more proud of my body for giving me my son," she wrote in an Instagram post.
"It was day one back at work (I'd been working from home, but this was my first commitment that required leaving the house – any new mum knows this is a big deal!) and Arnold was with me in the studio."
It was also the first time she had returned to exercising where she could only manage five minutes of training with low-impact modifications.
"But my smile says it all! I got it in," Tiff said proudly.
Tiff previously explained she took it "very slow" after giving birth, which wasn't what people expected of the fitness expert.
"It could have damaged my business because it changed my image and all that. But I didn't care. I wanted to rehab my body after the trauma of birth properly," she said.
The 34-year-old, who is married to comedian Ed Kavalee, said she tells new mums all the time not to bounce back but to bounce forward.
"Your body isn't the same anymore and that's OK. Mums need to take time to heal your body and build up stronger."
