The couple made a visit to the Alcoa Aluminium Smelter Project in October 1985, while in Australia for the state of Victoria's 150th anniversary. But when Diana and Charles were given hard hats at the factory, the Princess could not hold back her laughter at her husband.
Charles was given a white hat with “HRH Prince of Wales” written across the front.
Diana had a matching hard hat, but footage of the visit shows her holding back laughter over her husband's appearance.
The footage shows Charles catch on to Diana’s giggles, and then joke with the workers at the plant.
He asked: “Do your wives always laugh at you when you've got your hat on?"
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One said: “I remember Harry saying in the documentary, how his mother had the most amazing laugh. She surely does!”
Another added: “This lady was extraordinary, yet so ordinary in many ways.”
Others praised the royal for making “a hard hat fashionable” and called her “joyful”.
Other royal fans were disappointed in Charles for being “insecure” about the joke, and mocked the Prince.
One said: “He was annoyed and embarrassed! He even mimicked her laugh! Such an insecure bloke.”
Another added: “Probably laughing more because she can’t ever picture him doing a hard days work.”
One user pointed out Diana may have broke a royal protocol, and recalled reading about her giggle at Charles’ hat in a newspaper at the time.
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Footage of Diana playing a “notoriously complicated” piano recitals in Australia also re-emerged recently.
While the Princess was in Australia for her third visit in 1988, she played a section of Russian composer Sergei Rachmaninoff's Piano Concerto No 2 whilst visiting a school with Prince Charles.
Diana and Charles made the visit to celebrate Australia’s bicentenary, and spent ten days in the country to mark the occasion.