An Australian mum turned entrepreneur has created a sustainable underwear brand designed specifically for tweens and teens, and the story behind it is as compelling as the product itself.
Unlike almost every other brand on the market, Billie Green Underwear contains no polyester or nylon in the fabrics, making it not only supremely comfortable but a better choice for the environment and children’s health.
The idea came about when Caroline Hartline started consulting to a friend’s lingerie business and was shocked to realise nearly all intimates were made from nylon or polyester, essentially 100% plastic. Around the same time, she was struggling with secondary infertility and started researching the impact of fabrics on physical health.
“I was wearing a lot of activewear, something I didn’t do when trying to conceive my previous children, so I wondered if the fabric was affecting my fertility,” Caroline said. “When my baby was born, I started checking fabric labels and saw polyester and nylon were staples in baby clothes and young girls’ crop tops and bras. I knew that our children needed better options.”
The environmental case is just as urgent
The environmental impact was equally front of mind for Caroline. Research shows Australia is the highest consumer of textiles per person in the world, with more than 200,000 tonnes of clothing ending up in Australian landfills each year, most of which break down into microplastics.
“Especially with this new wave of Shein and Temu, fast fashion has become a huge issue. I’m really passionate about ocean conservation and the environment,” Caroline said. “My children have skin sensitivities that are exacerbated by wearing polyester and synthetic fibres. So all these elements came together and resulted in Billie Green.”
What’s in the range
Billie Green Underwear is designed specifically for tweens and teens with simplicity, comfort and sustainability at the fore. The crop tops, bras, singlets, undies, jocks and socks are all made from high quality natural fibres including Lenzing, TENCEL™ Lyocell and Organic Cotton. All products are certified to the OEKO-TEX® Standard 100, meaning they have been tested for harmful substances and are safe for sensitive skin.
The products have been designed from scratch to be seamless and supportive. Crop tops feature a longer length so girls won’t outgrow their bras as quickly, and bras feature interchangeable straps, as these are usually the first part to show signs of wear. Easy sizing means there’s no need for embarrassing bra fittings or complicated cup sizes.
“We don’t use any padding because it’s made from one of the worst types of plastics you can get and it traps heat. We use double layered fabric for breathable coverage without the chemicals,” Caroline said.
Why the fabric against your skin matters
Studies have revealed plastics are harmful to human health, potentially impacting reproductive and endocrine systems, child neurodevelopment, nutritional, circulatory and respiratory systems, skin-related disorders and cancer outcomes.
“When it comes to underwear, that fabric is sitting directly against your skin, which is the largest organ in the human body. The shedding of microplastics while wearing them as well as washing them is a huge health and ocean issue,” Caroline said. “I’m not interested in cutting corners or using cheaper materials. I sampled 200 fabric swatches and spent more than two years to get this brand right.”
Already a hit with kids and parents
Billie Green launched in August 2025 and is already generating up to $20K a month with a high customer satisfaction rate. Only two products have been returned, and the vast majority of customers repurchase.
Hundreds of young Aussies are raving about the comfort, and parents are loving the simple, minimalist designs. One of the current best-sellers, particularly during footy season, is the Boys Natural Skins / Light Compression Shorts, worn under rugby, AFL and soccer shorts.
“Almost every other option on the market is made from 100% plastic polyester blends,” Caroline said. “Ours are crafted from a premium TENCEL™ Lyocell blend, giving boys the same moisture-wicking performance but with a fabric that’s softer, cooler and far more breathable.”
What’s next: tackling school uniforms
Caroline isn’t stopping at underwear. She is now expanding into school uniforms, on a mission to remove polyester from school dress codes and offer a more natural alternative. A major concern is the chemicals in anti-stain and anti-wrinkle textile coatings applied during manufacturing, which don’t wash away.
“I’ve started the Uniform Project so schools can partner with us for uniform consultancy, uniform shop management, fabric education and subsidised pricing to make high quality natural fibres affordable and accessible,” she said. “We’re about to launch a pre-order for organic cotton polos for my children’s school so I am hoping this is the start of a change for schools across the country.”
For more information visit billiegreen.com.au
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