Prison phone call shows the breakdown of Cassie Sainsbury's relationship with her fiance.

Cassie Sainsbury’s imprisonment is starting to take a toll on her relationship, with the 22-year-old telling her fiance Scott Broadbridge she’s not sure they’ll make it.

A phone call aired during Channel Seven’s Sunday Night program showed mounting tensions between the young couple who have not seen each other face-to-face since Sainsbury was arrested carrying a large quantity of cocaine in a Colombian airport last month.

“We’re not going to last,” Sainsbury told her Broadbridge – who has flown to Bogota from South Australian support her – over the phone.

“Can’t you already tell?”

He accuses her of pushing him away, which she countered saying, "You're pushing me away."

Meanwhile, Channel Nine's 60 Minutes reports Broadbridge has been named a "person of interest" by Colombian prosecutors investigating the source of the 5.8kg of cocaine found in Sainsbury's luggage at Bogota's El Dorado International Airport on April 12.

And during his unpaid interview with Sunday Night, which his partner's lawyers in Australia sought - and failed - to have blocked from being broadcast last night, he repeatedly defended her despite a number of inconsistencies in emerging from her story, including with regard to the former personal trainer's work history.

Why are we so obsessed with Cassie Sainsbury's story? (Post continues...)

"Everything is telling me I should just walk away," Broadbridge told reporter Denham Hitchcock but vowed to stay with the Adelaide woman who risks 20 years behind bars if found guilty at trial.

"I'll wait. She would do the same for me," he said.

"I am willing to be there through to the end, and I've told her that. That's how I feel."