Olivia Newton-John shared the devastating news that she's battling breast cancer for the third time back in November.

And now the Grease star, 70, has admitted she doesn't know how much longer she'll live as the chance of a cure for her condition - which is at stage four - is very slim.

Speaking to 60 Minutes , she said: "When you're given a cancer diagnosis or a scary illness diagnosis, you are suddenly given a possibility of a time limit.

"If you believe the statistics, you're going to make them happen. If somebody tells you, 'you have six months to live', very possibly you will - because you believe that."

Olivia Newton-John is determined to 'win' third battle with cancer (
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REX/Shutterstock)

Olivia decided it was better for her mental health if her doctors didn't tell her how long she had left, as she's chosen to put her efforts into cancer research.

She is selling some of her prized possessions, including Sandy's leather jacket and trousers from Grease, to raise money for researching cannabis as a treatment for breast cancer.

The film star has been using cannabis grown in her home in California to help treat her symptoms.

Olivia Newton-John as Sand Olsson in Greece alongside, Jeff Conway as Kenickie, John Travolta's Danny Zuko and Stockard Channing's Betty Rizzo (
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Mirrorpix)

Olivia explained: "When I broke my sacrum, I was on morphine because the pain was intense.

"I was terrified of starting it because I know it's a hard thing to ween off but I am now off it and I weened myself off it with cannabis."

In September last year she revealed the breast cancer had spread to the base of her spine.

The shock diagnosis comes five years after her secret second battle with cancer and two decades after she first overcame the disease in 1992.

Olivia Newton-John is battling cancer for a third time (
Image:
Getty)
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In January she was forced to deny she was "at death's door" as she recorded a video for fans saying she is "doing great".

In a video released on Twitter , she said: "Happy New Year everyone, this is Olivia Newton-John and I just want to say that the rumours of my death have been greatly exaggerated, to quote a very famous quote.

"And I am doing great and want to wish all of you the happiest, healthiest 2019 as possible and thank you all for your wonderful love and support for me and for my Olivia Newton-John cancer wellness centre in Melbourne, Australia.