The Kath and Kim star announced she had stage-four mantle cell lymphoma, describing it as a "rare and fast-moving blood cancer".
Szubanski went public with the diagnosis in an Instagram post on Thursday in which her head was shaved, saying it was in preparation for her treatment in which it would fall out.
In her typical, positive style, the 64-year-old smiled and laughed throughout her video, at one point cackling as she exclaimed "what are you gonna do?"
"I won't sugar-coat it, it's rough," the 64-year-old wrote of her diagnosis.
"But I'm hopeful. I'm being lovingly cared for by friends and family, my medical team is brilliant, and I've never felt more held by the people around me."
According to the Leukemia Foundation, mantle cell lymphoma affects the outer-edge - or mantle zone - of B-cells in the lymph node follicle.
It accounts for about five to 10 per cent of all non-Hodgkin lymphomas, with the average age at diagnosis between 60 and 65 years.
Former Australian of the Year Grace Tame led a string of well-wishers in comments on Szubanski's post.
"Sending you all the love in the world and warm, immuno-safe virtual hugs," she wrote.
TV presenter Lisa Wilkinson said all of Australia was behind the comedian in her fight.
"Sending love, strength and every ounce of joy you have given all of us over the decades, to help see you through these difficult times darling Magda," she wrote.Â
Others sending love included entertainers Rove McManus, Peter Helliar and Julia Zemiro along with model Megan Gale and singer Dannii Minogue.
Szubanski said she would be lying low while her immune system "takes a hammering".
"I've been feeling pretty ratshit for ages. So I asked for extra bloods and - voila," she wrote.
"So the takeaway is, get tested and listen to your body."
Szubanski is best known for winning the hearts of Australians as the sporty, unlucky-in-love Sharon Strzelecki in sitcom Kath and Kim.
That sparked character roles in the smash-hit movies Babe and Happy Feet (and both sequels) among others.
She first burst onto Australian screens in the mid-1980s after being picked up by ABC talent scouts watching her perform in a university revue.
Her credits include sketch shows Fast Forward and critically acclaimed Big Girl's Blouse, alongside Kath and Kim co-writers Gina Riley and Jane Turner.
The comedian was a prominent marriage equality campaigner after coming out as gay in 2012.
She used her profile to push for a yes vote in the 2017 Australian referendum on same sex marriage, including a pivotal appearance on the ABC's Q&A program and an address to the National Press Club.
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