Police are exploring a number of theories on what caused a house fire in Castlecliff last week, including that the fire was accidentally started by a homemade lantern.
Detective Senior Sergeant Neil Forlong confirmed that was one of the theories they were looking at, "but it's not the only one".
The fire on Seafront Rd badly burned two houses on Sunday night.
Mr Forlong said forensic evidence was hard to obtain when everything was so badly burnt, "so proving exactly what it was is difficult, but we're working on a couple of thing at the moment".
Whanganui fire service chief Bernie Rush said though investigators had spent many hours looking for the cause, it likely won't be known given the damage and conditions.
"The house has been tested for accelerants and tested for electric faults but because of the weather conditions and the amount of water we put into the house it would have been impossible to find any residue."