If you've been searching for period swim underwear for your daughter and found yourself going down a rabbit hole comparing brands, you're in good company. It's a genuinely confusing category and the options have grown a lot in the last few years.
This guide breaks down three of the most searched Australian period swim underwear options side by side: Eco Period FlowBase, Knicked period swim, and Eltee Sydney period swim undies. We're an Australian-owned period care brand, so we're not pretending to be neutral, but we've done our best to represent each product based on their published specs, so you can make the right call for your girl.
Period swim underwear is worn under (or instead of) regular swimwear to absorb menstrual flow in the water. Unlike pads, which become waterlogged and useless, a good pair of period swim undies uses a water-resistant outer layer to keep pool or ocean water out, while the absorbent inner layers work to capture and hold flow.
The key things to compare are: how fast the inner layers absorb, how much they hold, how the outer fabric behaves in water, how visible the product is under swimwear, and how comfortable the fit and seam construction is during movement.
| Feature | Eco Period FlowBase | Knicked BFF | Eltee Sydney Underswim |
|---|---|---|---|
| Water-resistant outer | Yes | No | Yes |
| Fast-absorbing inner | Yes | Moderate | Yes |
| Absorbent zone width | Wide and extended gusset | Standard gusset area | Wide, extended zone |
| Seamless/bonded edges | Bonded edges, gusset and protection extends to the edges. | Gusset bonded to the inside. Small gap between gusset and edges. | Bonded smooth seams |
| PFAS-free | Yes | Yes ("free from nasties") | Yes |
| Shape and fit | Bikini cut with 6 cm side edges | Bikini cut with less than 5 cm sides | Bikini cut with approx 6 cm side edges |
| Worn under swimwear | Yes | Yes | Yes |
| Australian brand | Yes, Sunshine Coast QLD | Yes, NSW | Yes, NSW |
| Female founded | Yes, | Yes, | Yes, |
| Price (swim undies) | $28 | $33 | $36.95 |
| Size range | G10-12 - W16 | XS–20+ | G10-12 to W18 |
The FlowBase swim undies were designed for girls who want serious protection without anything showing under their bikini bottoms. A few things to note about Eco Period FlowBase swim undies:
The outer fabric is water-repellent, meaning water beads off the surface rather than soaking through. That matters for two reasons: it keeps the product feeling lighter in the water, and it means the absorbent layers inside aren't competing with pool water, and remain available to catch your period flows.
Inside, the FlowBase uses a fast-absorbing technical layer that draws flow in quickly and locks it into a wide protection zone, that extends right to the edges of the swim undies, locking in any leaks without overflow. The seamless construction means no rigid edges that can rub or leave a line under swimwear.
Eco Period is 100% Australian and female owned, and has been making period care products since 2018, before most of the category even existed in Australia. FlowBase is PFAS-free, reusable and washable, and built to last and perform.
[Shop FlowBase Period Swim Underwear]
Knicked is an Australian tween and teen period wear brand. Their swim range uses the same layered technology as their underwear range, but with nylon outer fabric.
The BFF (Bestie Flow Friend) is a sold as a hybrid product to be worn under swimwear or for everyday wear. This is due to the seamless construction and fairly thin gusset which makes it non bulky under tight clothing like swimmers or dance wear/ leotards etc.
Eltee Sydney is teen period wear brand based in Erina, NSW. The UnderSwim was their first release into the period swimwear market, with the brand starting out selling period undies for teen sports in particular. UnderSwim is stated to be designed as an under-layer worn beneath existing swimwear, with a bonded edge construction that keeps it invisible under regular swimwear.
There's no single right answer because it depends on what she needs:
If she swims competitively or has a swim carnival and needs something completely invisible under a race suit, any of these three will work, with Eltee having the most specific endorsement for that context.
If she wants something to wear under her own bikini for a beach day or pool hang, all three products are designed for that.
If she cares about PFAS-free and Australian made, all three brands meet that bar.
If she has a heavier flow or needs wide coverage during active movement, FlowBase and UnderSwim both specifically call out wider absorbent zones.
If organic materials matter to your family, Eco Period's broader range includes organic cotton options across the period underwear range (the FlowBase is [confirm material]).
If your daughter has just started her period, or is about to, period swimwear and swim underwear can genuinely change how she feels about going to the beach or pool. A significant percentage of girls avoid swimming during their period. That's a lot of fun, sport and social connection they're missing.
All three products in this comparison exist to solve that problem. The Eco Period First Period Kit also includes a guide to swimming on your period, alongside her first set of period underwear, which is a helpful starting point if you're navigating early periods with a daughter who swims.
[Explore the First Period Kit]
Eco Period was founded on the Sunshine Coast in 2018 by Danielle Hoeksema, a mum who wanted better options for her own family. We were one of the first period underwear brands in Australia and the first to insist on organic cotton in the gusset and outer fabrics of our underwear. You can read more about why here. All of our products are PFAS-free, reusable and designed to last. We're 100% female and Australian owned, and we ship from the Sunshine Coast.
