Whanganui Station officer Jes Sorensen said an unattended pot on the stove ignited the fire, and when the heat built up the fire took hold.
Mr Sorensen said had there been a working smoke detector the woman would have been alerted earlier to the heat from the fire.
"Smoke detectors need to be checked that their batteries are still working."
The woman was oblivious to the fire until woken by the banging of windows breaking. She was ready to ask her son, who lived in a bach on the section, to be quiet. They were the only people on the property.
She went to investigate the noise and saw the house was well alight.
She immediately ran to her mobile phone back in the bedroom to call the fire department. They arrived in seven minutes.
Yesterday she called in to work to say she would not be there, and instead spent the day salvaging what she could.
The woman, who has lived in the house since 1989, had just finished renovating the lounge and kitchen, and the house had also been rewired.