Pregnant Madeline Holtznagel steps out in a blue bikini as she joins Justin Hemmes for lavish Boxing Day yacht party near their $100m mansion

Madeline Holtznagel and Justin Hemmes celebrated Boxing Day in style as they hosted a luxury yacht party for their friends in Sydney.

The couple, who are expecting their first child together, put on a loved-up display as they cuddled on board his $320,000 TechnoHull Sea DNA 999 speedboat.

Madeline, 27, looked beach ready in a stylish blue bikini, made up of a plunging strapped top and high-waisted thong bottoms with floral detailing.

She shielded her eyes with a pair of oval shades from Miu Miu and carried her essentials in a straw handbag for the sun-soaked day out at sea.

Madeline placed her hand on her pregnant stomach at one point as she stepped out just weeks after her baby news was first revealed.

After boarding the lavish boat, Madeline cosied up to Merivale boss Justin, 52, as she was seen wrapping her arms around him in a sweet moment.

Justin showed off his muscular frame as he went shirtless in black board shorts while hosting their friends on his speedboat for the Boxing Day festivities. 

The group were seen boarding the high-end vessel near Justin's $100million Vaucluse mansion, where he and Madeline plan to raise their unborn child. 

One of Justin's friends has moved out of the eight-bedroom compound - which has been in the Hemmes family for more than 40 years - to make room for the baby. 

Justin, who has a net worth of $1.39billion, already has daughters Alexa, nine, and Saachi, eight, with his ex-girlfriend Kate Fowler, from whom he separated in 2018.

The business mogul then began dating Madeline the following year, when the model was 22 and he was 47.

In a huge step for the couple, their baby news then emerged last month, with sources telling Daily Mail Australia that they are set to welcome their first child together. 

Justin and Madeline's representatives told Daily Mail Australia at the time that the couple will not be making an official comment.

They have been spotted out together in recent weeks looking as loved-up as ever and mostly recently attended Pilates queen Bernadette Fahey's wedding to Jordan Sukkar.

Justin and Madeline quietly began seeing each other in 2019, a year after his debut on the Financial Review Rich List with an estimated net worth of $951million - which has since risen to $1.39billion.

Though they generally keep their romance under wraps, the couple are often seen disembarking from his sea plane after enjoying luxurious family holidays with his two daughters.

The couple have gone from strength-to-strength since going public and even sparked engagement rumours last June.

Eagle-eyed fans noticed Madeline was wearing a ring on her wedding finger as they questioned whether Justin had popped the question.

A picture showed her sporting the jewellery as she cuddled up to Justin while enjoying a bowl of Udon noodles together at a restaurant in Japan.

But the bling appeared to be an emerald and diamond promise ring, and another photograph showed her wearing a different ring on the same finger as she posed for a selfie.

Justin is considered Australia's hospitality king, with more than 80 venues and billions in assets with names like the Ivy, Queen Chow and Establishment under his purview as CEO of Merivale.

He is the only son of Merivale Group founder and nightclub mogul John Hemmes and Justin took over the business after his father's death in 2015.