The Sixth Sense star was aware there was a question mark over her family history, and she agreed to a DNA test as part of her appearance on genealogy show Who Do You Think You Are?

When the results came back, the actress was informed her dad's father Harold is not an ancestor, seemingly confirming suspicions her grandmother Norma MCWhinney fell pregnant during an affair with an American serviceman in the midst of World War II.

Collette is now appealing for help in tracking down the mystery man, who she believes is pictured in a photograph she has uncovered.

Appearing on the programme, the actress showed the image to viewers and said, "I think (this) is my grandfather. All I have is an image of him. I don't know his name. He was American. I know he was involved in World War II. He was in the navy. He spent some time in Sydney in the mid-'40s. If anyone has any information about this guy, I'll kiss you. Please come forward with it."

She adds to the Herald Sun newspaper, "Dad has never been sure who his father was and I was hoping to uncover that."