Australian singer Olivia Newton-John, who gained worldwide fame as high-school sweetheart Sandy in the hit musical movie "Grease," died Monday, her family said. She was 73.

Her husband John Easterling said she died peacefully at her ranch in Southern California surrounded by family and friends.

She had battled breast cancer for many years but a specific cause of death was not given.

Newton-John is best-known for starring in the 1978 musical "Grease" alongside John Travolta, as the-girl-next-door Sandy, who trades her ankle-length skirt and prim and proper hair for skin-tight black pants and a perm.

The high school sweetheart-turned-bad girl resonated with audiences worldwide, and continues to capture hearts decades after the movie was released. 

 'You're the one that I want'

Several songs from the movie, particularly "You're the One that I Want," "Summer Nights," and "Hopelessly Devoted to You" remain all-time favourites.

"Making it was fun but you never know with movies if audiences are going to go with it or not, even if you love it," she said in a Forbes interview in 2018. 

"It is incredible that it is still going but it's not even just that, it's showing no signs of stopping. You say "Sandy and Danny" and people instantly know what you're talking about."

Olivia Newton-John with John Travolta in Grease.Olivia Newton-John with John Travolta in Grease.

Grease remained the highest-grossing musical for three decades, with Newton-John and Travolta maintaining a close relationship long after the film was made.

"She was my favourite thing about doing Grease," Travolta said in an interview to mark the film's 40 anniversary in 2018.

There was no one else "in the universe" who could play Sandy, he said of Newton-John, who turned 29 during the making of Grease and later revealed she had to be convinced by Travolta to take up the role after self-doubts that she was too old to play a teenager. 

"If you were a young man in the 70s..., if you remember that album cover with Olivia with that blue shirt on, with those big blue eyes staring at you," Travolta recalled. 

"Every boy's, every man's dream was: 'oh I would love for that girl to be my girlfriend'."

Her career would span from singer and actor, to author and philanthropist in the coming decades, with her passion for cancer research at the forefront, championing natural therapies, including medicinal cannabis in the treatment of cancer.

She performed into her late 60s, until her latest diagnosis, including a two-year residency in Vegas, a 2015 tour with Australian music legend John Farnham and even recording a Club Dance track at 67 with her daughter Chloe Lattanzi.

"I have done everything, and the icing on the cake as well," she said, reflecting on her career. 

"So I feel grateful for anything that happens now." 

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