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The sisters' father, Jason Alexander, announced his younger daughter's death on Facebook on Friday 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 that went over a bank and burst into flames on Summit Rd on the Port Hills.
Tayla died at the scene and Sunmara, an Ashburton College student, was critically injured, including severe burns all over her body. Others in the car escaped serious injury.
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Sunmara was flown to Auckland's Middlemore Hospital where her condition continued to deteriorate.
Her father, who raised his daughters on his own since their mother moved to Australia 11 years ago, wrote on Friday his daughter was not expected to survive the night.
"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.
After Sunmara died, 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."