AN INSTAGRAM model who was found dead on a billionaire's luxury superyacht on Friday posted a cryptic message two weeks earlier.

Sinead McNamara, 20, had been working as a crew member on the £108million Mayan Queen IV for the past four months before her sudden death.

In a cryptic message a fortnight ago she wrote: "My head is all over the shop today,” along with emojis of a volcano, a tornado and a needle with blood dripping from it.

Sharing an image of herself on a quad bike, she added: “Take me back to this where my only worry was not cracking my skull open.”

Police in Greece have started a murder investigation after her body was discovered on the back of the huge boat.

The vessel, called Mayan Queen IV, is owned by Mexican billionaire businessman Alberto Bailleres, who News.com.au reports is worth around £7.4 billion.

Mr Bailleres, who has earned his wealth in the mini trade, reportedly left the yacht anchored off the island of Kefalonia on Tuesday.

Ms McNamara, from Port Macquarie in northern New South Wales, was left alone with the rest of the crew since then, reports say.

An autospy will be carried out on her body.

The 92-metre-long cannot leave the Kefalonia port until police have finished their investigation.

Australia's Department of Foreign Affairs and Trade (DFAT) said it has offered support to the McNamara family.

"DFAT is providing consular assistance to the family of an Australian who died in Greece," a spokeswoman said.

But the model's devastated brother Jake McNamara expressed his sadness on social media.

"I have unfortunately received the worst news of my life, (Sinead) has passed away," he posted on Saturday.

Family and friends revealed Ms McNamara, who has more than 14,000 Instagram admirers, was "jet skiing around the most beautiful turquoise water in the Med" just days before her death.

"It was only a few days ago you were messaging me forcing me to come camping with you. You're such a gorgeous girl lost way to soon," a friend wrote.

Another added: "Our souls were so perfectly aligned we were convinced we were the same person. I can't explain how much I'm going to miss you or how broken I am."

It is believed Ms McNamara moved from Port Macquarie to Sydney around five years ago.

Fellow Aussie and Instagram star Alex Hayes posted: “Life is so precious gone was too soon. Such a beautiful human being. Will cherish all our memories. Rest in Peace you beautiful soul”.