Ashburton Volunteer Fire Brigade chief Alan Burgess said the home on Suffolk St in Hampstead, Ashburton, did not have smoke alarms.
When firefighters arrived about 2am on Sunday the bedroom where the fire had started was fully involved, as was an adjacent room.
Flames were coming out the windows. The two rooms were destroyed and the rest of the house was badly smoke damaged.
There were a couple of variables in determining what could have caused the blaze.
As a result, the reason for the fire was unlikely to be determined.
Meanwhile, later on Sunday, fishermen at Pendarves beach lit a firework in tinder-dry conditions.
The Pendarves and Rakaia volunteer fire brigades then had to respond to the grass fire about 4.30pm.
"It had travelled 500m by the time we got there," McArthur said.
There had been a "reasonable north-easterly wind" which fanned the fire.
The fishermen had admitted to finding the firework and lighting it.
"They waited there and owned up to it."
Firefighters were at the scene for about an hour.
The dry conditions across Mid Canterbury have eased after rain this week but the area remains under a restricted fire season.
