"We had a report of a large explosion near a construction site just before 3pm,'' said Murray Hurst, police southern communications shift inspector.
Firefighters were on the scene within minutes.
"There was an explosion of a type of gas bottle,'' said Fire Service southern communications shift manager Riwai Grace.
Police evacuated the area and set up a 200-metre cordon.
Firefighters, helped by specialists from local gas companies, checked the scene and ensured it was safe before everybody was stood down.
Gas specialists and fire safety investigators are looking into the cause of the explosion. The type of gas bottle involved had not been released by officials, Mr Grace said. The Ministry of Business, Innovation and Employment (MBIE) has been informed.
Less than 15 minutes after the blast, just a few blocks away, emergency services were called to a serious car crash.
Two women, a 40-year and a 20-year-old, were trapped in the car and needed to be cut out before being taken to hospital with minor to moderate injuries.