Breast imaging there indicated a suspicion of cancer but a biopsy came back negative and the woman was diagnosed with plasma cell mastitis.
The woman's condition deteriorated and eventually a further biopsy was undertaken more than two months after her initial presentation, showing inflammatory breast cancer.
The cancer was aggressive and the woman died in 2019.
In today's report, Commissioner Morag McDowell found further imaging and biopsy should have been recommended.
McDowell was critical of Hutt Valley DHB for diagnosing the woman with plasma cell mastitis without questioning the biopsy result, which was not in line with the imaging results.
She also found the lack of a single clinician in charge contributed to the misdiagnosis.
"Due to multiple clinicians involved, the woman's care was affected by the lack of clarity as to which DHB and clinician had overall responsibility for her," McDowell said.
"Under the Code, consumers have the right to co-operation among providers to ensure quality and continuity of services, and therefore I have found that both DHBs are equally responsible for the delay in her diagnosis."
McDowell said Wairarapa DHB should have had a system to "red flag" abnormal results to clinicians, and said it was vital DHBs had systems for alerting clinicians to abnormal test results.
"(Wairarapa DHB) had the information needed to make an accurate diagnosis and provide the woman with appropriate care, yet its system failed to ensure that the information reached the appropriate clinicians within an appropriate time," she said.
"This contributed to an unnecessary delay for diagnostic results in a time-critical situation."
McDowell recommended the DHBs provide a written apology to the woman's husband and said both had made changes to their processes.
Wairarapa had created a new role to improve continuity of care for general surgery patients, and incorporated a system for flagging abnormal results into its patient records upgrade.
Meanwhile, the Hutt Valley DHB was improving its services and having discussions with Wairarapa about breast patient transfer and management.
"It is encouraging that both DHBs have acknowledged the lack of clarity in their breast service and implemented changes to their processes," McDowell said.