The music-industry giant will be honoured by local and international performing greats at a blockbuster charity gig at Melbourne's Rod Laver Arena on Sunday, raising funds for the cancer support services that helped him through his own battle with the disease.
The 77-year-old, who has undergone several surgeries for oral cancer, now requires continuous oxygen support and 24-hour care.
A day before the sold-out event, Farnham's wife Jill and the couple's sons Robert and James, said they were "deeply moved by everything happening" at Rod Laver Arena.
"John's health has deteriorated in recent months," read a family statement posted to social media.
"He now requires continuous oxygen support and uses a wheelchair for mobility.
"It has also become necessary for John to receive 24-hour nursing care at home, ensuring trained support is always available when he needs it.
"We have kept these matters private while our family has adjusted to John's changing needs.
"However, we now feel it is important that John's fans and supporters understand his circumstances and why he cannot be there this weekend."
The charity event sold out within hours of going on sale, with premium tickets priced at $1650.
The concert is being held to raise funds for Head and Neck Cancer Australia, which supported the much-loved singer when he was diagnosed with a cancerous tumour in his mouth.
Farnham underwent a 12-hour operation to remove the growth in 2022 and was later given the all-clear by his medical team.
The concert is expected to make a significant contribution to supporting Australians affected by these types of cancers, the charity previously told AAP.
Jimmy Barnes, Tina Arena, Human Nature, Keith Urban and Jessica Mauboy are among the names listed to appear at the gig.
The You're The Voice singer would have loved nothing more than to attend the concert, his family added in their statement.
"He would undoubtedly have been a little overwhelmed by all the fuss, but he would also have wanted to thank everyone involved in person," Farnham's family said.
"As a family, we have decided that it is best for John to remain at home, where he can be safe and comfortable.
"Thank you for your understanding and for the many messages of love and support John continues to receive."
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