Superintendent Starling said the dark, debris-filled river meant the divers could only locate the vehicle by feel.
Evacuation orders were issued for the town by state emergency services on Thursday.
"She was screaming [that] her mum, little sister and older brother had gone into the river in the car" he told the Seven Network yesterday.
Peter Taylor, a witness, told 9News: "A little girl's run down the road and she's said her family's gone into the river".
Donator Lynn Dunn-Marler wrote: "My small donation will never make up for your loss but I hope that it helps you and your daughter know that there are people out there supporting you in this tragic time".
Assistant Comm Loy said the child who escaped the vehicle and alerted emergency services was taken to Tweed Heads Hospital.
"The vehicle was completely submerged when we got there". I tried a couple of times.
Ms King's auto was located several hours later on Monday with sonar equipment about five metres from the northern bank of the river.
"It was just too deep, too cold, as soon as you opened your eyes underwater it was horrendous". "Then I saw the skid marks on the road".
He said the search effort is expected to continue into the evening.
It's the latest tragedy in a devastating few days for flood-ravaged northern NSW that has already claimed the lives of two women and a man.
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"At first we were going to go feet first, just pushing down to try and see if we could feel it with our feet".
Former police officer Matthew Grinham told of a frantic rescue effort on Monday afternoon, joining several others who dived into the freezing water of the Tweed and unsuccessfully tried to reach the vehicle.
Police said Tuesday that Stephanie had however managed to free her daughter, Chloe-May, who is now with her father Matthew Kabealo, after being discharged from Tweed Hospital on Monday night.
"There are barricades further up (the road) but they weren't on the roadway last night".
Mr Darcy said he and his partner were cleaning up mud from ex-Cyclone Debbie when the girl turned up dazed and bleeding early yesterday afternoon.
"Your mother... I can not imagine her loss, after losing your dad just two weeks ago. its beyond comprehension".
The scene of the tragedy at Tumbulgum.
After he gave up the underwater search, Mr Grinham took his tinnie out with a depth-sounder to make sure he knew the exact position of the auto.
NSW Police Assistant Commissioner Jeff Loy said the road the vehicle was travelling on was closed.
Dulguigan Rd has been closed due to flood damage since Friday but police are urging people to stay away from the area for their own safety.
"We really understand that people want to get to where they need to be, but this is an extreme event", he said. Emergency services were alerted about 1.40pm.