Inside the extravagant engagement party of real estate heiress Stephanie Onisforou and her entrepreneur fiancé Ari Kaklamanis

Sydney heiress Stephanie Onisforou showed off her stunning engagement ring at a swanky party on Friday, celebrating her upcoming nuptials with Ari Kaklamanis.

Stephanie, 25, the daughter of the property and retail tycoon Theo Onisforou, announced she was set to tie the knot to the 28-year-old Million Dollar Listings entrepreneur last year.

The designer behind the bride-to-be's impressive five-carat yellow diamond rock is Sophia Paspaley, a close friend of Stephanie's and member of the Paspaley family, famed for their pearl jewellery.

The festivities celebrating Stephanie and Ari's engagement took place at the bride's family estate, Kangaloon, in the Southern Highlands of NSW, reported The Daily Telegraph.

The exclusive guest list included Nicole Gazal O'Neil, a former cast member of The Real Housewives of Sydney, who enjoyed a sumptuous meal featuring oyster shuckers.

The publication reports that the dessert table was supplied by 'dessert designers' Strawberry and Co. and featured a cake with a swan decoration.

Details of the décor for the event included flags flying along the estate driveway and delicate frilled umbrellas on each table.

Photos have emerged of Stephanie sporting a bespoke gown from Sydney designer Lee Matthews for her milestone event.

Ari took a more casual approach for his outfit, which included a white shirt and chinos. 

It's understood that the happy couple have been together for several years.

So far, the pair, who are famously private, have not released any details or a date for their wedding.

Stephanie, who founded the French patisserie Cafe de la Fontaine in Potts Point, is a well-known figure among Sydney's elite with powerful connections.

Her father, Theo, was the chief money man for the late media mogul Kerry Packer.

He later moved into the real estate sector, where he would make serious money acquiring and revitalising unique and neglected buildings.

Among his achievements as a developer, he is credited with transforming Paddington's famous shopping district The Intersection. 

The tycoon has since diversified from property and retail into large-scale cattle farming.

Theo shares Stephanie and her twin brother Angus, with his ex-wife, Heidi. 

A well-known socialite and noted property whiz in her own right, Heidi set a national record after selling a Potts Point terrace for $13 million in 2016.

Stephanie, who sold her Potts Point shop recently, had previously worked in social media management.

Her brother Angus studied business and law at the University of Technology Sydney.

Stephanie has also been described as a socialite to watch, though we know less about her private life than Angus, whose exes include the OnlyFans star Indi Thew.

The Onisforous family have another daughter named Emerald, who keeps a lower profile.