"The lady was in bed. It's possible her dog woke her up, but she hasn't been able to get out. I don't think she got out of her bedroom. The dog died as well," Mr Bain said.
The woman would not be named until close relatives overseas had been told of her death.
Whangarei senior fire officer Paul Thompson said the dog was found in the hallway near the woman's bedroom. The bedroom door was open.
He said the fire was slow burning and probably started after a discarded cigarette butt ignited in the lounge. The woman's family had confirmed she was a heavy smoker.
Mr Thompson said it appeared none of the smoke alarms in the house were in working order.
He said it was a tragic reminder for people to ensure they had smoke alarms and they were operating properly.
Police are also investigating the woman's death, but would not be drawn on the details.
According to Fire Service statistics, half of all people who die in fires are careless with their cigarettes, matches or lighters.
Statistics also show that in 80 per cent of the fires attended in New Zealand, smoke alarms were either not installed or not working.