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The sisters' father, Jason Alexander, announced his younger daughter's death on Facebook last night.
"It is with great sadness I tell you this, my beautiful baby girl passed away at 8:27 tonight," he wrote.
Sunmara (16) and sister Tayla (17) were passengers in a car which went over a bank and burst into flames on Summit Rd, on the Port Hills.
Tayla died and Sunmara, an Ashburton College student, was critically injured, including receiving severe burns all over her body. Others in the car escaped serious injury.
Sunmara was flown to Middlemore Hospital where her condition continued to deteriorate.
Earlier last night, her father, who raised his daughters on his own since their mother moved to Australia 11 years ago, wrote that his daughter was not expected to survive the night.
"We have made her comfortable, and she has all the cards every one has kindly sent her for her birthday.
"It's been a long hard fight but this was a fight she couldn't win, there was no more they could do for her. Her injuries were just too much."
Last week, Alexander told the Herald he had forgiven the driver of the car. He also shared his message to teens behind the wheel this summer - don't speed.
Last night on Facebook, the grieving father thanked everyone for their support.
"It is truly what's kept me going, quite often when I'm feeling really down and start losing my strength I grab my ph and read all msgs you have sent us and it has picked me up again."