The health update was delivered in a comedic video uploaded to social media on Tuesday, with Lucy sharing a "not so much fun update".
"Long story short, I've got breast cancer," she said.
"If you've been wondering why I haven't been posting much personal stuff the last couple of weeks. Here's your answer. Strap in."
The comedian said she received an update from BreastScreen Australia about three weeks ago, telling her that results from a mammogram were "a bit weird".
"I went on last Friday for a lumpectomy, and a doctor, I'm not making this up, called Doctor Fun found more cancer," Lucy said.
"Doctor Fun is amazing. I am in her debt, but we have gone from a lumpectomy to a mastectomy, which I am having Friday week."
"I think I'm quite fine with going bra-free for my last chapter, and my last chapter is going to be very long and healthy," Lucy said.
The beloved performer announced she was giving up comedy - pub gigs and big, stressful stand-up tours - in December 2022.
Just a few months later she found herself with the main part in Samuel Beckett's Happy Days, generally regarded as an absurdist masterpiece.
Lucy most recently published a book, Well Well Well … My Wellness Odyssey earlier this month.
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