When the first crew arrived the woman was already safely outside.
The house was beyond saving so firefighters in breathing apparatus focused on making sure the flames didn't spread to neighbouring homes.
The structure of the house was still standing but it had been gutted inside.
The woman was checked at the scene by St John medics.
Paihia firefighters got back to the station at 5am.
Fire investigator Gary Beer said the fire had been caused by a super-heated wheat bag.
After heating up the bag in the microwave the woman placed it on a couch and left the room.
When the smoke alarm went off a short time later she returned to the room to find the wheat bag and a cushion it was resting on were on fire.
Unable to put it out, she triggered her medic alert and sought safety outside.
Beer said with repeated use wheat bags could become extremely dry and combust if they became super-heated.
He advised people to place a cup of water in the microwave with the wheat bag when heating it up to ensure it retained some moisture.
He also advised against leaving the hot bag on anything flammable.
Another fire in Paihia about two years ago had a similar cause though in that case damage was limited to the microwave.
Paihia fire chief Rex Wilson commended the woman for doing everything right.
''She did all the right things. She had working smoke alarms and she got out and stayed out,'' he said.
