The expansion of Waitakere Hospital's emergency department has almost doubled the size of the west Auckland facility.
The new building has been officially opened, but will not receive patients for another two weeks while staff get used to the new space.
Waitakere's emergency department has seen the greatest increase in patient numbers in the country.
The department's clinical director Willem Landman said in 2009, staff treated about 28,000 patients a year, but that had grown to more than 50,000 last year.
Dr Landman says waiting times will be reduced with the new facility, and he also hoped treatment times would be shorter.
"It's not just the time that you wait before you're seen by a doctor but also the time you wait after seeing a doctor to complete all the care that's required.
"And the way that this department's being designed is to ensure that we can deliver the care as soon as possible."
Mr Landman said the new building was necessary to meet the demand of a growing population in West Auckland.
Earlier this year, urban Māori authority Te Whānau O Waipareira Trust urged its whānau in west Auckland to boycott an after-hours clinic and go straight to Waitakere Hospital for treatment.