A new mum's inspirational “nana” pants Facebook selfie highlighting society's body-shaming problem has gone viral after resonating with thousands of women.

Australian Mel Rymill stripped down to her “nursing bra and nana undies” to share her powerful message about placing health ahead of appearance.

She acted after her personal trainer assumed that she was working out in a bid to go back to the weight she was before giving birth.

“It wasn't a question, it was a statement,” she writes. “And it pissed. Me. Off.”

The 33-year-old, who had been advised by doctors to stop exercising during her high-risk pregnancy, simply wanted to regain her strong body.

“I'm not worried by how my body looks, only how it functions,” she explains in the post, which has been shared more than 7,500 times on Facebook.

Criticising “harmful labels” and the pressure society puts on mothers to return to their “pre-baby bodies,” the media and community engagement officer from Adelaide called on women to join what she calls her #badassundies movement.

“What we should be worrying about is if people are ok, not what they look like,” she says.

Mel's post in full:  

So I had my first session with a PT today and the first thing she said to me was "Obviously you want to get back to your pre-baby weight". It wasn't a question, it was a statement. 

And it pissed. Me. Off. 

I corrected her nicely by simply saying "my goal is to regain my core strength and endurance...I'm not worried by how my body looks, only how it functions...it can be pretty badass". 

But it got me thinking. Post pregnant women are told they look good if they return to their pre-baby body quickly leading to the assumption that they look bad if the keep the extra weight. Skinny people are envied for their lack of fat or shamed for apparently starving themselves. Voluptuous women are either labelled fat and shamed or they're labelled brave for being comfortable in their own skin. There is always pressure. 

No one is comfortable in their own skin 100% of the time. Constantly labelling people and piling expectations associated with these labels on them is harmful to everyone...including those doing the labelling. 

What we should be worrying about is if people are ok, not what they look like. 

So here I am. I may not be magazine ready, my nana undies and bedtime nursing bra are certainly not going to be rocking a runway anytime soon, my hair is greasy, I have no makeup on, my body is squishy and plentiful, I'm not even sure I'm totally ok. 

But I am strong. My body is healthy. 

Hell, I am badass as fuck!

Screw what society wants from me. This is what's on offer. 

Join me if you will #badassundies”

Responding to the overwhelming support for her post, Mel explains to NBC that while she initially felt disheartened by the PT's comments, she remembered to be thankful for her “amazing body”.

“I looked in the mirror and started getting down on myself before snapping out of it and remembering that my body is amazing,” she says. “It allows me to do amazing things.”

Image: Facebook/Mel Rymill