The actress, 36, has revealed that she’s set to lose her hair and won’t be able to carry children in the future.
Keltie played Michelle Scully, the sister of Holly Valance’s character Felicity, on the iconic Australian soap from 1999 to 2004.
Her pal Ebony Gilbert has set up a GoFundMe page, which has so far raised just over £25, 000.
It reads: "Our beautiful Kate is going through the unimaginable. She was recently diagnosed with Stage 4 Breast Cancer.
"Her cancer is extremely aggressive and it has spread to other parts of her body, including her lymph nodes, blood and hip bones (known as metastatic cancer).
"This week the fight of her life begins as she starts chemotherapy."
The money will be used to cover Keltie’s rent, medical appointments, wig and specialist skin products.
Gilbert added: "Our independent Kate would never ask for help, let alone financial assistance.
"However, we know it’s what she will need, so here we are.
"Being unable to work and earn an income during this time has been a tough blow.
"By chipping in together, we can cover some living costs and try to remove financial stress off Kate’s plate (cancer is enough, thanks world!)"
“She will lose her gorgeous hair and cannot carry children in the future.
“This is devastating news for anyone, let alone at the age of 36, with no family history of breast cancer.”
“Kate will then require chemo infusions every three weeks for the rest of her life, and potentially radiotherapy and surgery too.”
Keltie said in a message on the GoFundMe page: "It’s so amazing to know I have so many incredible people in my corner.
"I’ve just got over Covid and I start my first round of chemotherapy tomorrow, and you really have no idea how my head space has changed going into it because of each and every one of you.
"Thank you, from the bottom of my heart. Kate x."
The page has attracted widespread support from Keltie’s native Australia and the UK.
Keltie has kept a low profile since she quit acting in 2008 and has been working as a legal consultant.