Now in his 70s, Thompson has been confronted with a stark choice - told by his doctor to lose weight, or else.
Heeding the advice, Thompson has turned his life around.
In both film and television his shadow has always loomed large, but one year ago Thompson's sheer physical size threatened not only his career but also his life.More stories from Today Tonight
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"I was obese. I was carrying this enormous weight. My back gave out, my knees gave out. The general lethargy (meant) I took a nap every afternoon - my poppy nap as my grandson liked to call it," Thompson said.
His weight had become impossible to ignore, and he felt a long way removed from the larrikin sex symbol Australians first met in the 70s.
He went from centrefold to the silver screen, and Thompson's performance in Breaker Morant brought him worldwide acclaim.
It was just the beginning of a brilliant career which has spanned over four decades, and having achieved so much, being reduced to breathlessness, crippling pain and constant lethargy was a nightmare.
The life of Thompson had become a physical and mental struggle.
"They say fat people are always happy, but I wasn't always happy. It did get me down because it felt like I'd lost all this energy and focus in a lot of ways. It made it very difficult for me to work," Thompson said.
Filming Baz Luhrmann's epic Australia, Thompson struggled with a physique at breaking point. Shortly after a severe hernia requiring an operation brought his weight into sharp focus.
"The doctor wouldn't operate until I lost ten kilos," Thompson said. It was literally 'do or die'.
"Somebody said 'what motivated you?' I said 'fear'. If that hernia had strangulated, I would have been a dead man."