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.