Emergency services were called to the blaze on Beechworth St about 5.15am on Saturday.
Two crews arrived to find the lean-to garage burning and fire spreading into the roof of the house.
Fire and Emergency New Zealand investigator Mark Bredenbeck said firefighters climbed into the roof and extinguished the flames.
The lean-to was completely destroyed, and parts of the inside of the house and roof were damaged.
"They managed to stop it spreading into the house too much, which has saved a lot of the owner’s contents, which is really good."
Crews stayed on the scene until late morning to be sure the fire was completely out.
It was unknown at that time whether the property had smoke alarms, he said.
"Basically the cause is unexplained at this point. We’re working alongside the police in relation to what’s happened."
Many items stored outside were destroyed in the fire, including household goods and books.
