In what is one of the Territory's worst Outback tragedies, the quartet left Willowra on Friday destined for the community of Jarra Jarra.
The trip is roughly 114km, or a five-hour drive on the Jarra Jarra Access Rd.
The couple's car broke down roughly halfway through their journey.
Weather records show temperatures in the area reached 39.9C on Friday, 41.3C on Saturday and 40.3C on Sunday.
On Wednesday, a passer-by came across the bodies of the 19-year-old couple and their son.
It would turn out they would be discovered 4.5km from their abandoned car, where remnants of food and water were found.
The "distraught" passer-by rushed to the Willowra health clinic to raise the alarm.
On Wednesday night, family members informed police that a 12-year-old boy from the same community.had also been travelling with the young family, sparking a desperate aerial and ground search for the missing passenger.
Tragically, less than 24 hours later, NT Police confirmed the boy's body had been found only 120m from the perished family.
Police are yet to rule out the possibility that the car was carrying yet more unaccounted for passengers.
"Although police have no information that suggests there were other passengers in the car, we will continue a precautionary land and air search until we are satisfied that all areas of interest have been sufficiently canvassed," Southern Desert Division Superintendent Jody Nobbs said.
Supt Nobbs said the man who initially raised the alarm at Willowra Health Clinic would be a critical part of the police investigation.
He said the tragic incident was a sad reminder to take precautions when travelling on remote Northern Territory roads.
"This does serve, unfortunately, as a timely reminder for anyone travelling on NT roads to ensure that when you travel your vehicle is in a fit state, that you have sufficient supplies including water and a first-aid kit, and that someone is aware of what your travel itinerary is – when you intend to leave, arrive and your anticipated routes," he said.
