The family of 69-year-old Gail Ellison are hoping to find the Good Samaritan who saved her life in Blockhouse Bay.
Ellison has type 1 diabetes and was found unconscious around 4pm on Monday and taken to hospital for urgent treatment.
"She suffers hypos and she's hypo-unaware," Ellison's daughter-in-law Allison Holden told the Herald.
"She remembers coming out of the hairdresser's and trying to start her car but it wouldn't start and that's the last thing she remembers."
Falling unconscious behind the wheel of her parked car with the windows up, Ellison spent about three hours roasting in heat from the sun.
According to MetService, Auckland reached a high of 25C on Monday.
However, a passerby discovered her car all steamed up beside the footpath, with Ellison still inside, and opened the door.
Ellison "vaguely" remembers travelling in the ambulance on her way to hospital and believes a man came to her aid.
"The doctor at the hospital said it was basically like leaving a baby or a pet in the car during that heat and she began to cook," Holden said.
"She doesn't know if she was imagining it or not but she thought it was our neighbour - which it wasn't - and she thought maybe I'm making it up.
"But then on one of my [Facebook] posts, a woman said she saw a man helping her but she didn't know who it was."
In a bid to track down her mother-in-law's guardian angel, Holden posted throughout Facebook hoping he would see the post and get in touch.
"I don't think I'd still be here if it weren't for that man and I really would like to find him," Ellison told the Herald.
"I think someone up there was looking after me because I hate to think what might have happened if I had driven.
"I just can't get over how lucky I was ... I really would like to meet and thank him."