"I would travel around the country and stop into the country's best boutiques with my suitcase full of bikinis and clothes and a lookbook. I'd talk abut the brand and I think buyers really couldn't say no to me," Ms Rose O'Rourke said.
"They couldn't ignore how passionate I was about the brand and how much I backed it and I believed in it."
The brand's quality fabrics, flattering cuts and a chic, low-fi aesthetic soon became a hit with millennial women looking for an alternative to the mainstream brands like Seafolly and Tigerlily, which are constantly scattered across our beaches.
Part of their marketing strategy involved gifting bikinis to popular Instagram influencers, but it didn't hurt that actor Margot Robbie and models Gemma Ward and Nicole Trunfio were all papped wearing Zulu and Zephyr.
"Gemma Ward was a purchasing customer. She had some beautiful pieces in her feed that we stumbled across. She's since come on to support us by attending some of our events - not as a paid guest, but as a supporter of the brand," Ms Rose-O'Rourke said.
"When Margot Robbie wore our stuff, that was really exciting. She was doing some rehearsals at the Oscars and she was wearing Zulu and Zephyr to rehearsals and we found that on her Instagram page. Same with Nicole Trunfio, just true Australian girls who resonate with our message and our brand."
It is a truth universally acknowledged that in 2017, a beautiful, well-curated Instagram feed is as effective, if nor more so, than massive billboard ads or a prime time TV spot.
"Social media is a huge part of our business. As soon as we post something on our feeds, it's sold out on our online store and also in the boutiques. Its leverage is really powerful. But we have never paid someone to wear our product," Ms Rose-O'Rourke said.
She and Ms Rose take most of the photos on the Zulu and Zephyr Instagram page themselves, using old school film.
"Having original content is really important for us. We didn't want to repost anyone's imagery and just having an Instagram page that was about product. It was about continuing the mood of the brand," she said.
"It's not just about money, it's about the customer and her vibe. We have a really specific and loyal customer, girls who keep returning each season.
"We were coming in at a great price point and often customers would come in off the back of our social media. We had such a strong following ... our followers were going directly to our stores to try the garments on and get a feel for them."
But despite the celebrities endorsements, it was still a tough slug before the brand began turning over a profit.
"We didn't get paid for the first few years. We were reliant on working in cafes and still walking into those stores with the suitcase," Ms O'Rourke said.
"We went $20,000 into the red to begin with, but we made that back very quickly and everything we made we put back into the business. We felt a bit smug when we did get our first small wage.
"It's a multi-million dollar company, but we both receive an average wage because we are investing in our team and our brand.
"We were lucky to come into this at a time before there were dozens of other swimwear labels on the market, like there are now. We got in before that massive rise of the category."
The sisters now manage a team of 12 staff and are looking to expand further into the overseas market.
But Ms O'Rourke says maintaining the brand's low-key authenticity - a word advertisers use constantly when talking about marketing to millenials - is key to its success.
"It would be great to see Zulu and Zephyr hanging on the racks in boutiques all over the world and doing collaborations with those major online retailers," she said.
"I feel like in a modern society we always pay attention to who is wearing what and how they wear it. The brands we wear these days say a lot about who we are as people. Are we more laid back? Are we more feminine or masculine?
"It tells a story about who you are, so when someone is wearing a Zulu and Zephyr piece, it says they're a certain personality."
