In a statement on his website, Michael said the cancellation of nine concerts “breaks my heart”.
He confirmed he would still fulfill his UK dates in October but would then undergo treatment for his condition.
Michael suffered from life-threatening pneumonia and spent a month in a Vienna hospital last winter.
The ex-Wham singer added in his statement: ” I have tried in vain to work my way through the trauma that the doctors who saved my life warned me I would experience.
“They recommended complete rest and the type of post-traumatic counselling which is available in cases like mine but I’m afraid I believed (wrongly) that making music and getting out there to perform for the audiences that bring me such joy would be therapy enough in itself.”
He has already performed numerous dates across Europe over the last month.
The singer added that despite enjoying his recent performances, “unfortunately, I seriously underestimated how difficult this year would be”.
The nine cancelled concerts were due to take place in Perth, Adelaide, Sydney, Melbourne, Brisbane and the Hunter Valley.
The Symphonica tour was resumed this autumn after it was pulled when he became ill in November last year, just hours before he was due to perform in Vienna.
He had completed 46 of the original 65 dates when he contracted pneumonia.