The mystery over what happened to an Australian model found dead on a billionaire's superyacht in Greece is deepening after a chilling message came to light.

Two weeks before a Sydney model was found dead on a billionaire's superyacht in Greece, she posted a cryptic message to Instagram.

My head is all over the shop today," Sinead McNamara wrote, 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."

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Sinead McNamara's post has raised concern following her death. Photo / Instagram
Sinead McNamara's post has raised concern following her death. Photo / Instagram

The chilling post has emerged along with new details about Ms McNamara's mysterious death on board a billionaire's superyacht — and they raise more questions than answers.

The 20-year-old had been working on the Mayan Queen IV for the past four months before her sudden death on Friday.

The luxurious US$140 million, 92-metre yacht is owned by Mexican mining magnate Alberto Baillères, who is worth an estimated AU$9.6 billion.

According to local media, Mr Bailleres left the yacht, which was anchored off the island of Kefalonia, on Tuesday — leaving Ms McNamara alone with the crew. Her body was reportedly found on the back of the boat two days later.

What happened in the 48 hours after the billionaire's departure, in the lead up to Ms McNamara's death, is now the subject of an inquiry by local authorities.

"The Coast Guard of Kefalonia has ordered an investigation into the circumstances that led to the death of the 20-year-old Australian national as well as an autopsy-necropsy," In Kefalonia reported.

Sinead McNamara's family has confirmed that she sadly passed away on holiday in Greece. Photo / Instagram
Sinead McNamara's family has confirmed that she sadly passed away on holiday in Greece. Photo / Instagram

The Mayan Queen was scheduled to set sail today but has been ordered to remain docked in the port of Argostoli until detectives have interviewed all crew members and a forensic examination of the vessel.

In a tragic development, local reports suggest desperate attempts were made to revive Ms McNamara at the scene before she died on route to hospital.

"Local port authorities were informed at 2am Friday morning that the body of the woman was found … at the back of the yacht," Kefalonian Mantata said.

The paper said officers from the port authority, crew members and a doctor "tried for long time to bring her back" before rushing her to a hospital in Argostoli.

Doctors made the decision to fly her to a private hospital in Athens but Ms McNamara died during the helicopter flight, local media said.

The Department of Foreign Affairs and Trade 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.

Sinead shared on Instagram she was a fan of travel and adventure. Photo / Instagram
Sinead shared on Instagram she was a fan of travel and adventure. Photo / Instagram

Originally from Port Macquarie, Ms McNamara moved to Sydney five years ago.

She was travelling in the seaside town of Pylos on the southwest coast of Greece.

In her last Instagram post four days ago, Ms McNamara wrote about jetskiing in Komitata, Greece.

Ms McNamara had been working on yachts while travelling in Europe.

Just five days ago on Facebook, Ms McNamara posted a selection of photos with her big sister Lauren, writing "happy birthday to the most amazing big sister I could ever ask for, 5 days till I get to see you!!! Excitement is an understatement."

There has been an outpouring of grief of Ms McNamara's Instagram profile, from fans of her travel and modelling photography.

"Rest In Peace you gorgeous girl, life is so unfair," wrote @lilyrobinson.

A number of people have also posted comments asking about the circumstances of her death.

Ms McNamara writes on her Instagram page that she is a fan of "camping, 4WD, adventure and travel". She has 12.7k followers on the platform.

Australian YouTube star Alex Hayes posted the news to his social media accounts.

Hayes, who has more than 650k followers on Instagram posted the tragic news in an Instagram story, writing "Life is so precious gone was too soon. Such a beautiful human being. Will cherish all our memories. Rest in Peace you beautiful soul".