A Fire and Emergency New Zealand spokesman said firefighters attended after an alarm activation at 12.15am and found a carbon dioxide leak.
Once that leak was made safe the building was ventilated.
A McDonald's spokesman said staff evacuated the restaurant just after midnight when a safety alarm was triggered.
A small gas leak was located and fixed, allowing the restaurant to re-open about an hour later.
"Staff and the fire service took the necessary precautions to ensure everyone was safe, and the systems in place meant the issue was able to be dealt with quickly and effectively.''