"We have recently discovered that John has a cancerous growth," his family, including wife Jill, said in a statement obtained by AAP on Tuesday.
"He has been admitted to hospital this morning for surgery and ongoing treatment."
In a separate statement provided by the family, Farnham said a cancer diagnosis was something many people face each day "and countless others have walked this path before me".
"The one thing I know for sure is that we have the very best specialist healthcare professionals in Victoria and we can all be grateful for that. I know I am."
The family has requested privacy at this time.
Farnham sang his way into Australian hearts as a fresh-faced teenager in the 1960s but had faded into near-obscurity before Whispering Jack shot him back to the top some 20 years later.
The 1986 album produced one of the nation's most well-known anthems, You're the Voice, and propelled Farnham to hero status.
Farnham followed up Whispering Jack with his chart-topping albums Age of Reason (1988) and Chain Reaction (1990).
Farnham was born in 1949 in Dagenham, England, and aged 10 emigrated to Australia with his family, who settled in Melbourne.
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