A woman with cancer had a relapse of hepatitis and required a liver transplant as a result of failures by the Capital and Coast District Health Board.
A Health and Disability Commissioner report shows the woman had a history of hepatitis B and took antiretroviral medication to prevent it recurring because cancer treatments suppressed her immune system.
But it relapsed when systemic issues at the DHB meant the medication, lamivudine, was stopped early.
The intention was she would take it for a year after chemotherapy.
Commissioner Morag McDowell was critical about the woman only being given verbal information about the need to take her medication.
This information was not reinforced through pamphlets, clinic letters or medication labelling, nor was she or her GP given a treatment summary, McDowell said.
"The DHB lacked clarity about roles and responsibilities, and there was no formal protocol for preventing hepatitis B reactivation in patients undergoing immunosuppressive therapy," said McDowell.
There was "no clear plan" to ensure the woman stayed on lamivudine following chemotherapy.
"As a consequence, her prescription for lamivudine was stopped too early and this went unnoticed resulting in her hepatitis B being reactivated."
McDowell recommended the DHB use an anonymised version of the report as a case study during education sessions, as well as update the commissioner or changes it has made and apologise to the woman.