The fire officer investigating the blaze that destroyed one house and badly damaged two others in a central Auckland suburb says it was deliberately lit.
The blaze started in a house being renovated on West End Road in Westmere about 8.30pm on Saturday and quickly spread to two other properties.
More than 60 firefighters were needed to bring the blaze under control.
The area was cordoned off for much of Sunday as police and fire investigators examined the scene.
Bystanders told emergency services on Saturday night they saw several people running from the house, which was being renovated, before the fire started.
Specialist fire investigator Russell Dickson says it was deliberately lit and police are interviewing a number of youths.
Mr Dickson wouldn't comment on whether the fire was lit in multiple places or if an accelerant was used, saying that information is now part of the police investigation.
An old bungalow which residents say had been purchased only six months ago was gutted. Residents said it was empty as renovations were getting started.
Two neighbouring houses were badly damaged and two vehicles were burned out.
Margaret Lewis and Lisa Shearer were inside one of those houses at the time, and describe opening the door to find flames, several metres high, lapping at the side of their house.
Ms Lewis says by the time Lisa Shearer opened the door the flames were shooting through the roof and they were lucky to get out.
She says she's begun sorting through her ruined belongings but expects none of it is salvageable.
Neighbours told of pillars of black smoke and huge flames and of hearing explosions.
Claire Goddard whose house backs onto the properties said she heard blasts as the fire started. "We saw some very high flames and two very loud bangs, like a bomb really. We were very scared ... I was very pleased I knew where all my family were."
Mr Dixon told Radio New Zealand's Morning Report programme the house where the fire started was very old and dry, with no linings to help contain the blaze, which is why a massive fireball engulfed the building.
Firefighters initially feared people were inside the buildings but all residents were later accounted for. No-one was injured.
Residents were taking belongings from the homes on Sunday while groups of people watched from across the road and fire and police staff carried out their investigations.