Olivia Newton-John: Grease star, singer dies at 73

"The whole world will feel this heartbreak today because the entire world felt Olivia's unmatched light," she said on Instagram.

"A force for good. A force of nature. Strong and kind. My mentor, my friend, my inspiration, someone who always guided me. She was always there for me. Family to me.

"I don't have all the words I would like to say today but I hope everyone will join in celebrating our beloved Olivia, her heart, soul, talent, courage, grace. I love you forever."

Newton-John's death has resonated across the world, with celebrities, comedians, writers and politicians commending Olivia's work on the screen on stage and in her fight against cancer at the Olivia Newton-John Cancer Research Institute.

Her Grease co-star John Travolta also made a touching tribute to his "dearest" Olivia.

"You made all of our lives so much better," he posted on Instagram.

"Your impact was incredible. I love you so much. We will see you down the road and we will all be together again. Yours from the first moment I saw you and forever! Your Danny, your John!"

American singer Mariah Carey took to Instagram to share a number of photos of her and Newton-John over the years.

"Rest In Peace Olivia Newton-John," she posted.

Kylie Minogue said ever since she was 10 years old that Newton-John had been her inspiration.

"Since I was ten years old, I have loved and looked up to Olivia Newton John," she posted on Twitter.

"She was, and always will be, an inspiration to me in so many, many ways. My deepest condolences to her family and loved ones. x ONJ4EVER."

American singer and actress Barbara Streisand wrote on Instagram "too young to leave this world. May she RIP".

Well known Australian actor and author Magda Szubanski posted on Twitter that it was a "sad sad day".

"Thank you for all you gave us," she wrote. "Your beautiful soul will be so missed. My deepest condolences to John, Chloe, Gregg and Rod. Rest In Peace darling Olivia. I Honestly Love You".

American country singer Stella Parton also paid tribute, calling her an "angel girl" on Twitter.

"I have no words except to say my heart is broken. RIP angel girl and sing with the angels," she wrote.

Radio host and podcaster Geoff Quattromani said his favourite memories of ONJ was when she was touring with fellow Australian legend John Farnham.

"For many she'll be remembered for her role in Grease. For me it's the amazing tour and performances with John Farnham. Great memories," he posted on Twitter.

The Australian High Commission in London, Australia House, posted a tribute on its Twitter page for ONJ who was born in the UK and emigrated to Melbourne when she was five years old.

"We are terribly saddened by the passing of Dame Olivia Newton-John AC DBE. British born and Australian raised, Dame Olivia inspired us in song and brought us together as one," the post stated.

"We remember her with our British friends, and send our deepest condolences to her loved ones."

Former Australian Prime Minister Malcolm Turnbull has also paid tribute.

"Thank you Olivia Newton-John for your remarkable life," he posted on Twitter. "You lit up our lives with music and dance and you saved so many lives with your passionate and advocacy for cancer research and treatment."