Kayla Itsines

She also started training her sister's friends who wanted to improve their fitness so they could play Netball on their high school team, focusing on building core strength and powerful legs. She lectured them on nutrition, and asked them to take before-and-after photos of themselves to track their progress. Itsines' 12-year-old cousin suggested using Instagram as a way of organising the photos. Within a few months, Itsines gained thousands of followers and was receiving regular requests for advice and help. Her partner, Tobi Pearce, suggested that she compile her routines into ebooks that could be sold online. In March 2013, they founded the Bikini Body Training company with Itsines as director and Pearce as CEO. The following January, they published the first two Bikini Body Guides, with Itsines' workouts in one volume, and nutrition information from dieticians Julie Dundon and Anne Schneyder of Nutrition Professionals Australia in another. Itsines would later offer the guides via an affiliate program as well. By October, the guides had been downloaded over one million times.

In 2015, she embarked on a world tour, the "Kayla Itsines Bootcamp World Tour", leading free group fitness classes. The tour included Australia's capital cities, New York City, Los Angeles, and London.

In February 2015, Itsines took legal action against another Adelaide-based personal trainer, Leanne Ratcliffe, and Ratcliffe's partner Harley Johnstone over comments they had posted on their YouTube channels. Ratcliffe, publishing under the pseudonym "Freelee the Banana Girl", had claimed that the BBG eating plan required users to starve themselves, and Johnstone, publishing as "Durianrider", claimed that Pearce used steroids. Itsines claimed the statements were defamatory. She obtained an injunction to prevent the further publication of the videos. When the complaint was heard in the Supreme Court of South Australia on 23 March, Judge Withers directed the case proceed to trial, but the matter was eventually settled out of court.

In April 2015, Apple featured Itsines in an advertisement for the Apple Watch, demonstrating the heart-rate monitor feature of the product.

In November 2016, Pan Macmillan published Itsines' first print book, The Bikini Body 28-Day Eating and Lifestyle Guide. In August 2017, she announced the upcoming publication of her second book, The Bikini Body Motivation & Habits Guide.

In October 2016, Itsines and her partner, Tobi Pearce, were included in the BRW "Young Rich" list of the wealthiest Australians aged under 40 who had not inherited their money. Their net worth was reported at A$46 million in 2016. Itsines and Pearce's net worth was estimated as A$486 million on the Financial Review 2019 Rich List. Itsines' net worth did not meet the A$472 million cut-off for the Financial Review 2020 Rich List.

Itsines met her fiancé, Tobi Pearce, at a gym. They have lived together since 2012. In April 2018, they got engaged however announced their split on 21 August 2020.

Itsines gave up drinking alcohol and hasn’t had a drink since at least 2012. She gave birth to her and Tobi’s daughter Arna in May 2019.