Jason Alexander is living a parent's worst nightmare.

The father of two teenage sisters involved in a fiery car crash is due to head home to Ashburton for his older daughter's funeral - before returning to Auckland where his younger daughter is fighting for her life.

Tayla Alexander, 17, died when the car she was in went off the road on Summit Rd, on Christchurch's Port Hills, about 11pm last Wednesday.

The car crashed and caught fire. Tayla died at the scene.

Now her younger sister is fighting to stay alive after suffering critical injuries, including serious burns, in the crash.

Their father, Jason Alexander, has taken to social media to quell rumours that had been circulating about the circumstances of the accident.

"There was no other car involved. They weren't boy racing. It's just a tragic accident that has completely destroyed me.

The scene of the crash in which Tayla Alexander, 17, died late last Wednesday. Photo / Kevin Clark

"It destroys me more when I read and hear of rumours that are not true. Please respect my grieving and my family and friends grieving.

"These two girls were my life - my daughters and my best friends."

Jason Alexander said his older daughter had suffered impact damage to her head and it was thought she died quickly as a result.

"The other child in critical care is also my baby girl. She was trapped in the car and has had very bad burns," he wrote on Friday night.

"She is now in Auckland [City] Hospital undergoing surgery. She is in a very bad way and has a 10 per cent chance of surviving, but she is fighting really hard and isn't giving up."

'THE KID IS FIGHTING VERY HARD'

He said yesterday that his daughter had endured a long day and had come through it. She was due to have another operation today.

A doctor told him at one point that they were walking on a tightrope blindfolded.

"But because of the situation and the loss of Tayla, they are willing to go beyond what they normally would be doing, just to give us all that chance of seeing her again.

"She is still in a very bad way. The easiest way to put it is if she makes it through [the night], then that's amazing. If she makes it through [today's] surgery, then that's a miracle," he said.

"She has a lot of injuries and, unfortunately, because of the massive burns she has - a lot of the injuries she is suffering cannot be treated now because it will most certainly cause infection.

"She has no skin to protect her from infection forming."

It is understood at least two other teenagers were in the vehicle at the time of the crash.

Police are still investigating the circumstances of the crash.