Ivan Milat’s long-time mistress and sister-in-law said she will “always love him” and doesn’t want him to die.

Speaking publicly for the first time, Marilyn Milat-Tempest told The Daily Telegraph she worries about the serial killer’s health after he was diagnosed with oesophageal and stomach cancer.

“I don’t want him to die,” she admitted. “I don’t love him like that, anymore. Well … I always will. I worry for him, I call Long Bay jail almost every day to see how he’s doing.

“Ivan is Ivan, he’s different to anyone I’ve met. In his last days I can’t forget [him] … I worry for his health.”

The pair’s 11-year affair began in the 1960s. Together they had a child – Lynise, now 54 – when she was in a relationship with Ivan's brother Boris.

Ivan was arrested in May 1994, and sentenced to seven life sentences two years later for the murders of seven people between 1989 and 1993.

More than 10 years after their affair ended, Marilyn asked the backpacker killer to marry her when she learned that his wife Karen was divorcing her. Ivan rejected the proposal.

“Ivan was always at Marilyn’s house in Niagara Park, he was incredibly charismatic,” said daughter Lynise’s former partner Paul Gould.

“She loved it when he turned up … they’d sit in the bedroom talking for hours. She’s never stopped loving him. Lynise didn’t know for years that Boris wasn’t her father, he raised her as his own.”

Last month, Ivan was transferred from Goulburn’s super max jail in NSW to Prince of Wales Hospital in Randwick, Sydney, to undergo cancer treatment. 

The convicted serial killer, now 74, was moved to Long Bay Jail's hospital two weeks ago. He has been told he may only have weeks to live, and is expected to see out his days at the correctional centre’s hospital facility.