The 36-year-old actor - who played Michelle Scully in the Australian soap between 1999 and 2003 - has begun chemotherapy to treat the cancer, which has spread to her lymph nodes, blood and hip bones.
Keltie has thanked fans after a Go Fund Me page set up by friends raised over $31.5k AUD (£71.4k) to help support her through her treatment while she is unable to work.
The former soap star said: "I just wanted to say thank you, thank you, thank you for all of your support, and messages and phone calls. It's so amazing to know I have so many incredible people in my corner.
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"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."
He friends Rachael and Ebony said in a statement: "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.
Given the severity of the chemotherapy that Kate requires, her Oncologist advised that working and maintaining her job during this time, is not an option. 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."
The fundraising campaign has quickly surpassed its $25k AUD target to help pay for household bills and rent as well as medical care.
Keltie played the youngest daughter in the Scully family, sister to Holly Valance's character Felicity Scully.
She joined the soap in 1999 and left in 2003, only to make a brief return to Ramsay Street in 2004.
Since leaving Neighbours she has begun a new career in recruitment.