Chloe Kabealo said she had unbuckled her seatbelt and tried to "go up for air", then "just kept floating up out".
She said of her lost family members: "They were all loved and they'll never be forgotten."
Her father, who was not in the car, said he was "shattered" by the loss.
"I'm not holding up," Matt Kabealo said. "I'm just being strong for my daughter."
Chloe and her mother, sister and brother were in a car in the small town of Tumbulgum in New South Wales when it slid off a muddy road into a flooded river earlier this month.
Chloe escaped and ran to a farmhouse to raise the alarm.
Stephanie King, 43, died trying to save her children.
Local police superintendent Wayne Starling told reporters from 7 News at the time: "The mother was trying to get one of her children out of the car when she passed away.
"She was with the child, holding the child. I have no doubt she would still be alive if she wasn't trying to save her children."
Ella Jane, 11, and seven-year-old Jacob also died.
Chloe and Mr Kabealo were speaking at an event raising funds for them. So far efforts have gathered tens of thousands of Australian dollars.
"Anything we can do to make their lives a little bit better, we'll try anything we can," local policeman Constable Brad Foster told 7 News.
In March last year, a four-month old baby was the sole survivor when a car sank off the coast of Donegal in the Republic Ireland.
Advice for escaping a sinking car
If a car you are in starts to sink, get out as fast as possible. Do not phone for help or try to retrieve possessions. There is very little time.
Open the windows straightaway before contact with water makes the electric system fail or water pressure stops you winding the windows down.
If that doesn't work, get the door open, undo your seatbelt and get out.
The third option, in last resort, is to pull a headrest out and use the metal part of it to hit the window, hard, in the corner and hopefully break it open.
If you are underwater when you leave the car, push away from it, and if you don't know for sure which way is up, check what direction bubbles are floating in and swim that way.