As the world learns of the death of Olivia Newton-John, her Grease co-star, John Travolta, has paid tribute to his "Sandy", with whom he shared a special bond for more than 40 years.

Olivia Newton-John and John Travolta in a scene from Grease. Photo / Getty Images

Responding to the news that Newton-John had lost her decades-long battle with breast cancer, Travolta wrote on Instagram:

"My dearest Olivia, you made all of our lives so much better. 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 moment I saw you and forever!
Your Danny, your John!"

In their roles as tough guy Danny and sweetheart Sandy, Travolta and Newton-John's "electrifying" on-screen chemistry saw the 1978 musical film Grease become a cult classic.

And while rumours of an off-screen romance ran for years, Newton-John told Australian podcast Mamma Mia in 2018 "it just didn't happen".

"We were both with other people when we were filming and I think, respectfully, it just didn't happen," she said.

Newton-John instead described her relationship with Travolta as "a unique bond" born of their shared experience on Grease.

She explained: "We did something life-changing, making that film. [At the premiere] you got the feeling from the energy that something was happening. It was a huge response. I feel grateful to be a part of that and to have worked with him. We've stayed friends ever since."

Travolta has also said of their time on the film: "When you share that kind of meteoric success - and nothing has been able to exceed it - you share a bond. I've been through her having a child, getting divorced, losing her sister. She's been through my getting married, having children. It's wonderful and full of shared memories."

Last year Newton-John recalled her first on-screen kiss with Travolta, telling US Weekly she was definitely nervous but that it was "lovely" and "special".

"It was at the beach - actually, the opening scene of the movie.

"It was a beautiful day ... and an exciting moment because it was the start of the film, although everything was kind of out of order, but we did that. That was the first time we had kissed.

"I remember just before that we were sitting on a rock and it's actually in the movie that these waves broke over us ... And I can't remember if that was before or after that because my hair was perfect. So maybe it was after that.

"But it was a very special day and it was really a nice way to get to know John a little bit because we hadn't spent all that much time together."

The pair remained close friends who would work together again.

Olivia Newton-John and John Travolta remained life-long friends. Photo / Getty Images

In 2012 they released a Christmas album, bringing in guests Barbra Streisand, Cliff Richard and Tony Bennett.

And in 2019 they squeezed back into their Grease outfits - for the first time in four decades - for a special Meet 'N Grease movie sing-along event in West Palm Beach, Florida.

Newton-John excitedly wrote on her own Instagram page: "First time in costume since we made the movie ! So excited!! (sic)"

While Travolta shared on his account: "#Grease is still the word. Opening night of #MeetNGrease at the @coralskyamp in West Palm Beach! (sic)",

Newton-John also addressed fans who had long hoped for a Grease reunion movie:

"I think it's a bit late now," she said. "What would you do? Put us in a nursing home? I can't imagine that working now but you never know what someone could come up with. If there was a great idea then, of course, we would consider it, or at least I would. If John was up for it then I think I would be too but we couldn't do it without each other. Let's see what the future brings."