She said her only symptom was this small dimple and encouraged women to make sure they check for all abnormalities, not just large lumps.
"PLEASE PLEASE PLEASE check yourself and get your loved ones to check themselves also," she wrote.
"If I can help one other person, the way I was helped, then it's been worth showing my soon-to-be-reduced left tit to the world."
Ms Warner has since created a Twitter account called My Left Boob to keep her followers updated with her diagnosis and her original Facebook post has been shared more than 25,000 times.
From age 40, regular mammograms should be part of your breast health routine. The NZ Breast Cancer Foundation recommends annual mammograms from 40-49, then every two years after that.
Under 45, you'll need to pay for mammograms yourself - but seeing as one Kiwi woman a day is diagnosed under age 45 it can be money well spent. Shop around your local mammogram providers - prices can vary quite a lot.
If you're over 70 and still in good health, consider continuing with mammograms. Again, you'll need to fund them yourself, but given that nearly 25 per cent of breast cancers in NZ are found in women aged 70+ it could be well worth it.
The five lesser-known signs of breast cancer
• Changes in the skin of the breast, including dimpling, puckering or redness
• A change in breast shape or size
• Unusual breast pain
• Changes in the nipple, e.g. a turned-in nipple
• A nipple discharge
