The 47-year-old lived by himself in Wentworth Falls, and had been battling serious illness for some time.
His lawyer and friend, Charles Waterstreet, has confirmed the death, saying the artist had apparently died in his sleep.
Cullen was known for his portrait of actor David Wenham, which won the Archibald Prize in 2000.
He admitted completing the work in a matter of hours.
“It took a lot of time just staring at a blank canvas,” he said.
In total he entered the prize at least nine times, and was a finalist several times. He also won the prestigious Mosman Art Prize in 2005.
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