Sunmara Alexander, the surviving sister of a teen killed in a Christchurch car crash two weeks ago, has died in hospital on her 16th birthday.

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 Alexander. Photo / Supplied
Sunmara Alexander. Photo / Supplied

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.

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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.

"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."

Sisters Tayla, left, and Sunmara Alexander. Photo / Supplied
Sisters Tayla, left, and Sunmara Alexander. Photo / Supplied

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.

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"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."