Southern Cross insists its policy has not changed since September when it said in a statement that they "may be able to offer cover" on an ex-gratia basis.
But King says the letter she has been sent shows a clearer position.
"[Southern Cross] Health Society intends to add cover for diagnostic digital breast tomosynthesis as part of our annual benefit review process which next takes place in mid-2017," chief executive Peter Tynan wrote to King.
"However, until this time, the Health Society is providing ex-gratia coverage for members who have been recommended tomosynthesis."
King said the letter's contents were a positive thing for women all over New Zealand and represented a shift in the company's policy.
"Absolutely, I did actually show it when I met with the Breast Cancer Foundation a couple of days ago and they said it was fantastic," King said.
"The reply is what it is. If you hadn't raised the story, I probably wouldn't have even known about it.
"But this letter is good news."