Itsines has as combined social media following of more than 12.6 million (twice as many as Gwyneth Paltrow) and was described as "the Taylor Swift of sit-ups" in an article titled by Bloomberg.
She was just 18, and a new graduate from the Australian Institute of Fitness, when she realised the custom-workouts she created for her personal clients could reach a wider audience.
"I started uploading client transformations on Instagram, and then all of a sudden people started asking if I could come and train them from over the world," she told news.com.au in 2014.
"Obviously I couldn't get to everyone, so I started developing an exercise plan people would be able to do at home."
And thus the series of Bikini Body Guide e-books were born, and the uber-successful app followed.
Some criticised Itsines for using the term 'Bikini Body' to promote her business and in the Bloomberg interview she admitted she doesn't like the term BBG anymore, because it implies there's only one type of body that should be in a bikini.
"Do I regret calling my guides Bikini Body? My answer is yes," Itsines said.
"That's why when I released the app (in November last year), I called it Sweat With Kayla. Sweat is so empowering. I love that."
Who made the top 10 on the 2016 BRW Young Rich list?
Topping the BRW Young Rich list this year are Mike Cannon-Brookes and Scott Farquhar with a record $4.68 billion combined wealth after they floated their software company Atlassian on the NASDAQ exchange in the United States late last year.
They have topped the BRW Young Rich list a record five times, jointly taking out the number 1 spot in 2012, 2013, 2014, 2015 and again this year.
• Number three and four on the list are another technology duo in Dave Greiner and Ben Richardson, founders of Sydney company Campaign Monitor. Although they have stepped away from day-to-day management of the firm they remain directors and shareholders.
• Number five on the list is Perth property developer Paul Blackburne. His Blackburne Property Group dominates the Perth apartment market and develops three to four large apartment projects per year.
• Sixth is technology investor Simon Clausen, who is involved in thriving tech businesses such as Chinese real estate portal Juwai.com and Freelancer.
• Number seven on the list is Melbourne property developer Tim Gurner, who builds apartment projects in hip inner-city suburbs such as Collingwood and North Fitzroy.
• Eight is Swisse Wellness CEO Radek Sali, who heads a thriving business that has found huge success exporting vitamins to China, and is also starting to invest in start-ups himself.
• Number nine is New York-based technology and property identity Ori Allon. His real estate tech firm Compass has made a big splash disrupting the US property scene.
• Rounding out the top 10 is Sydney retailer and biotechnology company founder Sam Prince. He owns the Mexican fast food chain Zambrero and emerging biotech firm Life Letters, a genetics testing company.
