Helen Garner has won the prestigious Baillie Gifford Prize for nonfiction for what judges called her addictive and candid diaries.

The 82-year-old Australian writer was named winner of the Stg50,000 ($A100,300 prize at a ceremony in London for How to End a Story.

Journalist Robbie Millen, who chaired the prize jury, said Garner was the unanimous choice of the six judges.

Millen said the judges were captivated by the sharp observation and "reckless candour" of Garner's book, which covers her life and work between 1978 and 1998.

He said it is "a remarkable, addictive book. Garner takes the diary form, mixing the intimate, the intellectual, and the everyday, to new heights.

"There are places it's toe-curlingly embarrassing," he said. "She puts it all out there."

Garner's 800-page opus i

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