A report by Commissioner Carolyn Cooper found in 2018, a senior doctor failed to provide an acceptable standard of care and breached the Code of Health and Disability Services Consumers' Rights.
She also found the doctor was not experienced enough to work unsupervised, and did not seek advice from someone more senior or a cardiologist when he should have.
The woman, in her 70s, had collapsed after complaining of "crushing" chest pain and shortness of breath, and was taken to hospital by an ambulance.
At the emergency department, the senior doctor ruled out diagnosing an aortic dissection - a tear in the aorta, which is a "serious condition" - despite the woman's presentation and concerning symptoms, Cooper said.
The report showed the doctor ruled it out because the woman's condition appeared to improve, her pain lessened, and there were no "typical features" like "sharp, tearing or ripping chest pain and unstable vital signs".
He considered it could have been reflux, and sent the patient home with anti-reflux medication and advice that what she was experiencing was not cardiac related.
The next evening the woman's husband called the ambulance as she appeared to be having a seizure. At the hospital she was vomiting, pale and cold, had low blood pressure and was in significant pain.
The doctors were not sure what was wrong, but made a preliminary diagnosis of a gastrointestinal bleed, and a CT scan suggested she had an infected gallbladder. She was transferred to the intensive care unit in the early hours of the next morning.
She went into cardiac arrest just before 7am, and died at 7.30am.
A post mortem found she died of an acute aortic dissection with a fatal hemopericardium - an accumulation of blood in the space around the heart.
"Although this is a rare diagnosis, it is one with severe consequences and I'm not satisfied that appropriate steps were taken to allow the senior doctor to exclude this confidently," Cooper said.
The doctor also failed to use "appropriate tools" for discounting other relevant diagnoses, nor did they seek cardiology advice before discharging the woman, she said.
"I consider that had the appropriate review of the woman's history been undertaken, along with the advice from her family about the significant family history, it would have been appropriate for the senior doctor to have obtained cardiology advice prior to the woman's discharge."
Te Whatu Ora said the doctor had sufficient experience to work with minimal supervision - but the doctor admitted he needed more training. He had been in the role two months.
"I was very early on in my emergency medicine training but was given a senior position. In retrospect, I should have declined that position," the doctor told the commissioner.
He also admitted he should have consulted with another senior doctor before discharging the woman.
The report also found a "significant lack of documentation" of the woman's records.
The doctor said he was under considerable pressure at the time "due to the expectation that patients were to be seen and discharged from ED within six hours".
The doctor was individually accountable for his decisions and should write to the woman's family to apologise, Cooper said.
But she also noted Te Whatu Ora carried out a "serious adverse event review" after the woman's death - then lost the report. The health agency was also criticised for not assessing, monitoring and managing the woman's pain and vital signs throughout her time in hospital.
Since the woman's death, the doctor has educated himself further about detecting aortic dissection, and attended a meeting to discuss what went wrong, and how to avoid similar problems in the future.
And Te Whatu Ora has changed its system so doctors have three months of supervision to ascertain their experience before being placed on the senior doctor roster.
It also updated advice about when to escalate problems to consultants, and added sessions on chest pain and different diagnoses to its junior doctor training programme.
