The police say a fire that engulfed four houses in Lower Hutt, north of Wellington, on Sunday was deliberately lit.
The blaze began at a house under construction near the central business district.
Police say investigators have determined an accelerant was used to start the fire, which was discovered at about 1.30am on Sunday by a passing police patrol.
Twenty people were evacuated as strong winds spread the flames to neighbouring houses, causing extensive damage.
The fire service says nine appliances struggled to contain the fire.
The police want to hear from people who were near Cornwall Street in Lower Hutt early on Sunday morning.
Meanwhile, an elderly Blenheim man who died when his house caught fire at about 4am on Sunday has yet to be named.
The police are trying determine the cause of the blaze, but they say the man used his fire on most days to heat hot water.
