Australian fitness queen Kayla Itsines has announced she is engaged to her long-term boyfriend, Tobi Pearce.

The 26-year-old Adelaide-based personal trainer and entrepreneur, who has nearly 10 million followers on Instagram, uploaded a picture of her stunning sparkler to social media this morning.

"It was during a family dinner which included some close friends, and I really wasn't expecting it so my reaction was of total shock and happiness. It was really romantic as Tobi was on one knee," Itsines said in a press release.

"We have lived together for over 4 years, we have 2 beautiful dogs, an amazing business....and to top it off... Tobi got down on one knee last week and asked me to marry him.... in front of my family!," she wrote on social media.

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"I obviously said YES to spending the rest of my life with my very best friend. We have been together for 5 and a half years now, and I cannot wait for the many years and memories to come .... I love you so much."

Pearce said he managed to keep the engagement plans a secret despite the two living and working together.

"Kayla is really close with her family so it was important for us to have them there. It really made it a magical moment for both of us," he said.

Itsines is famous for being the creator of multiple successful fitness ebooks titled Bikini Body Guides.

The power couple have also formed the hugely popular 'Sweat with Kayla' app over the last four years, which helped launch their fitness business into a multimillion-dollar global empire.

Sweat became one of the top-rating and highest-grossing fitness apps worldwide.

The couple are estimated to be worth a whopping $46 million last year, according to BRW's Young Rich List.