The fire service was alerted by neighbours to Monday's blaze in Kells Ave, Aramoho, at 8.12am and arrived on the scene at 8.19am - but there was little they could do to save the house.
Fire risk management officer Shane Dudley said most of the damage would have been done in those seven minutes.
"A house fire like this doubles in size every 30 seconds," Mr Dudley said. "And this fire spread really quickly."
The Shemwell family, who rented the property, had just left when burglars struck. Police said the fire had started then.
Mr Dudley said one of the major contributors to fast-spreading fires was furniture.
"A La-Z-Boy has the equivalent of 10 litres of petrol in it," he said.
It appeared the house had no smoke alarms, and had they been installed it might have alerted neighbours sooner.
Families who did not have smoke alarms could phone 0800 NZ FIRE to see if they qualified for a free 10-year fire alarm worth $40, installation and a home safety check by firefighters, Mr Dudley said.
Anyone with information about the burglary is asked to contact Constable Reece Seivwright at the Whanganui Police Station on 06 3490600. Information can also be provided anonymously to Crimestoppers on 0800 555111.