Oamaru police sergeant Tony Woodbridge said the woman, who spent two nights in Oamaru Hospital with smoke inhalation and shock, was lucky to get out of the house.
Two units from the Oamaru Fire Brigade were called to the house about 2.20am and found it well alight.
It took about three hours to put out the fire, which caused major smoke and fire damage to all but two rooms.
The sole occupant of the house at the time of the fire did not want to speak to the Otago Daily Times.
However, her neighbour, Graeme Kerr, said the woman was asleep when the fire broke out and it was the cries of her cat that woke her.
"The cat saved her - it needs a wee medal," he said.
On Saturday afternoon, the tabby cat was curled up in the fence, oblivious to all the fuss.
However, he had been restless until then.
Mr Kerr was asleep when his neighbour banged on his front door.
"I thought it was some larrikins calling out. Then she said her house was on fire," he said.
When he looked out, he saw smoke coming from the front door.
Then the front window blew out and the flames took hold.
"It was an inferno in four minutes," he said.
Using the lessons he learnt from a fire-safety lecture at his workplace, Gillies Metaltech, he asked her if anyone else was inside.
Her fellow tenant was away.
"She was in shock and suffering from smoke inhalation. The fire brigade called an ambulance and she was taken to hospital," Mr Kerr said.
Mr Kerr and his wife were told by firefighters to evacuate their house, and he did not get back inside to bed until about 5.30am.
Sgt Woodbridge said the fire was not suspicious, but its cause was being investigated by Fire Safety.
The woman was being assisted by Victim Support, particularly to find new accommodation.
Oamaru chief fire officer Gary Gibson said the fire damaged most rooms, except for two which had their doors closed.
He said the house may be used as an educational tool because it clearly demonstrated how closing doors to rooms could help prevent a fire spreading.
