One man escaped the Marine Drive house uninjured.
Fire and Emergency NZ investigator Jason Hobbs said the blaze started in vegetation below the hillside property before catching fire to the deck.
He said the actual cause could not be determined. However, he said it was not suspicious.
Fire crews from Governors Bay and Diamond Harbour attended the blaze at 5.30pm.
Fire crews also attended another blaze on Karaka Rd in Hurunui, North Canterbury, which caused moderate damage to a sleepout.
Crews from the Hawarden and Karaka stations attended the fire about 5.30pm on Monday which was not deemed suspicious.
No one was injured or inside the sleepout at the time.