Two other people were in the property, divided into two flats, including one male who was upstairs asleep.
Flatmate Nicola Black, 24, said: "Good on her; she managed to get the mattress outside.
"Then she ran past it, while it was burning, into my flat and then told George, who was upstairs asleep.
"She was burned when she came running down to me.
"She's got pretty bad burns on her legs, arms and face, and she's just really shocked."
Mr Cullimore said the woman suffered potentially life-threatening injuries, including major burns and smoke inhalation.
Paramedics treated her at the scene before she was taken by ambulance to Lakes District Hospital in Queenstown. "We won't know until she's been properly assessed," Mr Cullimore said.
Three engines and 12 firefighters attended the scene after Miss Black raised the alarm with a 111 call.
The building has smoke alarms. Mr Cullimore said when the brigade arrived at about 12.50pm, the fire had gone on to the external wall.
"It started going up the wall. Two firefighters wearing breathing apparatus extinguished it using low-pressure delivery."
Ten people live in the two flats, but only three were at home.
First on the scene, photographer Blair Pattinson, said: "They wouldn't get out of the house at first. They were upstairs, the man and the girl with the burns, and couldn't get out the back because of the burning mattress.
"I yelled at them to get out of the house, over the balcony on to the adjoining roof.
"The fire brigade arrived to bring them down."
ODT er
