Te Whatu Ora Waitaha Canterbury Chief Medical Officer Richard French said the patient's condition deteriorated shortly after they left the ED last Sunday.
He said on returning to the ED the person was seen immediately and treated in intensive care.
"Sadly, the patient did not respond to medical treatment and died the following day," he said.
A formal review of the care provided to this person is underway.
"We acknowledge our ED was very busy during this period which meant wait times were longer than usual," French said.
"Our heartfelt sympathies remain with the whānau at this difficult time.
Last week, Health Minister Andrew Little confirmed no big city hospitals are meeting their target for treating emergency department patients within six hours.
His comments followed a report into Middlemore Hospital's ED that found only half of all patients at the department are seen within six hours, describing it as "dysfunctional, overcrowded and unsafe".
The report followed the death of a woman who left because of delays in being seen.
