How two young women on the cruise of a lifetime ended up behind bars in Sydney.

The Instagram account Melina Roberge and Isabelle Lagace share is now ‘private’ but the description still reads: “currently travelling”.

The account’s 56 posts document the pair’s journey across the world on a seven-week Sea Princess cruise, that disembarked from Southampton in England on July 9, 2016, and landed in Sydney on August 28.

From Ireland and the US to South America and New Zealand, Lagace, now 29, and Roberge, now 24, posed for pictures in bikinis and sunglasses, sipping on coconuts with tropical waters in the background.

It was the tip of a lifetime for the two Canadians, with Roberge at one point posting: “I used to be afraid to get out of my little town and now I feel like I don’t want to see that little town anymore cause it’s beautiful out there.”

But the wanderlust ended dramatically when Sydney police and customs raided the boat and allegedly found 95 kilograms of cocaine, distributed between four suitcases. The suitcases were allegedly stowed in the women’s cabin, and also the cabin of 63-year-old Andre Tamine.

The trio, all from Quebec, were arrested and believed to be working together.

The cocaine, which was allegedly divided into one-kilogram plastic bags and worth millions of dollars, signifies the largest bust by authorities in Australian waters, The Sydney Morning Herald reports.

According to customs officials, Lagace and Roberge had been considered "high-risk" passengers from the moment they boarded the "high-risk" ship (high-risk because it stopped at so many ports).

Months after their arrest, Lagace - a former porn star - pleaded guilty to importing nearly 30 kilograms of cocaine, valued up to $21.5 million.

But Roberge denied all knowledge, sobbing as she faced a judge, claiming she didn't know what was in the suitcases. Tamine, too, pleaded not guilty.

Only now, as prosecutors were preparing to take Roberge and Tamine to trial for drug importation, they have both changed their pleas to guilty.

Roberge issued her plea in a Sydney court on Friday, and could face a life sentence for the offence, The Canada Star reports.

Lagace told the court in November last year she agreed to smuggle the drugs so she could pay off her $20,000 debt. She attempted to justify her actions by saying she was acting under duress.

Although police do believe the pair were acting as pawns for a "very well-organised syndicate", Fairfax Media reports, the judge denied Lagace's claims of coercion.

"I am satisfied the motive was profit, whether the forgiving of a loan or financial reward," Justice Kate Traill said at the time.

Judge Traill sentenced Lagace to seven years and six months jail, with a non-parole period of four years and six months, and the term was backdated to August 28, 2016.

Isabelle Lagace jailed for more than seven years for trying to smuggle cocaine off cruise ship https://t.co/vUfwKLXQZD pic.twitter.com/aPhJbhRpoI

— MSNca (@MSNca) November 3, 2017

For close to two years, Roberge maintained her innocence, saying she didn't know what was in the bags and that she couldn't be linked to carrying the drugs.

There was no CCTV footage of her in possession of the bags, and her fingerprints weren't on the packaging.

(Lagace's fingerprints weren't on the packaging, either, which led Judge Triall to conclude she might not have been aware of the amount she was carrying.)

But, as reported in The Sydney Morning Herald, the circumstantial evidence against Roberge was damning.

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In December 2016, when magistrate Robert Williams told Roberge she would stand trial for the importation of drugs, he reminded the court:

• The women booked their trips within days of each other
• They listed the same person with the same email as their emergency contacts
• And that they had shared the cabin for at least 39 days.

"It's highly improbable that a person other than the Roberge or Lagace would have stored the items in the suitcases under the bed," Williams said at the time.

"It's also notable that the cabin space was tiny and the suitcases can be described as reasonably large suitcases."

Roberge is due to be sentenced on March 21, and Tamine will be sentenced on October 26. Both will remain behind bars until then.