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Investigative journalist Nick McKenzie has called revealing the truth behind Australia's most decorated soldier the toughest assignment of his 21 year journalism career.

Ben Roberts-Smith lost his defamation case against three newspapers who reported he had murdered civilians in Afghanistan.

McKenzie was the first to report the story - and has now released his new book Crossing the Line.

He told Mike Hosking no one - including himself - wanted to believe Roberts-Smith was a war criminal.

But McKenzie says while it's weighed heavily upon him - his stress in exposing the story is nothing compared to what the S.A.S soldiers faced in standing up to Roberts-Smith.

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