Three shops were destroyed and the fourth was badly damaged and may also be written off.
A faulty switchboard in Main Street Interiors caused the fire which spread to the other shops within minutes.
Insurance assessors said today it was yet to be decided if the fourth shop would be demolished or repaired.
They also refused to confirm a rumour in town that the damage estimate was more than $2 million.
Fire safety officer Craig Bain said the investigation team included fire safety officers, police and insurance representatives and they all agreed the switchboard caused the fire.
About 100 firefighters fought the blaze, and at its height, more than 20 appliances were in attendance from throughout the region.
Owner of Foster's Home Decorating, Ken Foster, said he watched helplessly as the business he and his wife Elain had worked hard to build up over many years became a blazing inferno.
The fire was the biggest in Dargaville for more than 40 years.
