The much-loved actor, 32, who played nurse Dan Goldman, is believed to have committed suicide, according to the Daily Telegraph.
Cast members of the hit Channel Seven medical drama are in mourning after the news was broken today.
Priestley was behind a huge ratings surge for the show on Tuesday night after his character finally married that of fellow star Jolene Anderson, after a lengthy on-off screen relationship.
The sudden death is the second recent trauma to hit All Saints, after actress Virginia Gay was mugged in Sydney – allegedly by two thugs accused of later killing a man.
A Seven Network statement described Priestley as “a tremendous young person – loved and respected by his fellow All Saints cast members and crew.”
Seven’s Director of Programming and Production Tim Worner described him as a “brilliant artist”.
“His work on stage and screen was admired by everybody who knew him,” Worner said.
“His death is such a sad loss. We at Channel Seven will miss his energy and smile.
“Our thoughts and prayers are with Mark’s family and friends.”
Priestley’s co-star and close friend Jolene Anderson is currently starring in the Andrew Lloyd Webber stage musical Tell Me On A Sunday.
It is unclear whether she will appear in tonight’s performance.
Priestley graduated from NIDA in 1999. He joined the cast of All Saints in 2004.
The official All Saints website biography for Priestley says: “Mark’s first big TV break was when he appeared in the The Farm in 2000 and met director Kate Woods. She gave him a key role in her mini-series Changi in 2001 and since then he’s been no stranger to television.”
His also appeared in Blue Heelers, and has worked with the Bell Shakespeare Company and the Sydney Theatre Company.
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