The Waitemata District Health Board is investigating how a nurse gave a patient about 10 times the medication she was prescribed.
Shirley Curtis, 60, died a day after being given about 100 milligrams of the beta blocker metaprolol at North Shore Hospital.
Ms Curtis was prescribed about one-tenth this amount in a standard chart.
DHB chair Lester Levy says the nurse who administered it was experienced and had a faultless record.
Dr Levy says the DHB will conduct a full investigation into what he describes as an "unfortunate error".
He says even with safeguards in place, mistakes do happen and it is up to the Coroner to decide if the error contributed to Ms Curtis' death.