Police and Fire and Emergency New Zealand attended the blaze.
The teen's grandfather was later transported to Oamaru Hospital, where he is being treated for smoke inhalation.
The small fire on the lounge floor was extinguished when a large rug was thrown over it, Sergeant Tony Woodbridge, of Oamaru, said.
He described the girl as "a bit of a hero".
If it was not for her, all four of the occupants might have perished if the blaze had spread, he said.
Asked about being labelled a hero, the girl said it "feels kind of cool", but was more worried about what could have been.
"It's hard to think about because it has not fully set in that there was a fire. It's just a bit scary to think about what could have happened.
"My granddad was sitting in front of it. He was a bit dazed I think."
Woodbridge said the incident was a close call.
"If it was not for the smoke alarm, we could have had four fatalities."
Oamaru Volunteer Fire Brigade Senior Firefighter Kevin Murdoch urged people to check their smoke alarms regularly, and to contact the brigade if they wanted them installed.
The cause of the fire was being investigated, but was not considered suspicious.