A police diver says John Breckenridge's car, found in Blue Cod Bay in the Catlins after the disappearance of him and his stepson, was "like a tin can that had been squashed".
Senior Sergeant Bruce Adams was involved in the search for 11-year-old Mike Zhao-Beckenridge and John Beckenridge in 2015.
At a coronial hearing into whether the duo are dead or alive, Adams described the challenge to deploy divers in the area due to large swells and dangerous conditions.
By the time the wreckage was able to be recovered, it had been in the water for more than a month, it was rusted, missing its bonnet and the seats had been "squashed".
Adams was asked in court on Friday: "If Mike and John had gone into the sea with the vehicle and the vehicle had suffered the kind of damage you saw, what would you expect to have happened to their bodies?"
"They'd be severely damaged just by the mechanism of injury, just by a fall from that height and that significant impact," Adams responded.
"With the nature of injuries that would be sustained in an impact like that it's going to accelerate that degrading of the person in the water, not just the water but the environment with marine life and mammals and the like.
"You're racing against time to recover… people intact in the ocean."
Adams was then asked what sort of deterioration could've been expected after nine days.
"Again, apologies to the family, it's parts of the body have been consumed, parts have been lost or broken off and lost," he said.
"Very soft tissue has gone, you're starting to end up with skeletal remains and sometimes that's all you end up with."
There was an oil slick of up to 40m radius in the bay, indicating where the vehicle was.
A search was carried out using a helicopter to lower a camera into the water.
Adams said the visibility was "less than half a metre" but divers were able to locate several vehicle parts.
Multiple attempts to dive in the area were made in the following days - dangerous conditions hampering each attempt.
On March 29, 2015, conditions were marginally better and divers were deployed.
When they found the vehicle, its bonnet was missing as well as all doors and windows.
"Front seats appeared to be reclined or pushed back due to roof impact. Both front seatbelts were clicked in and tight with lap belts tight into the folded seat," Adams said.
The purpose of the hearing is for coroner Marcus Elliot to rule whether they are dead or alive. Mike's mother believes her son is still alive and his disappearance was staged by her ex-husband.
On March 13, 2015, Beckenridge breached a parenting order when he went to his stepson's school and picked him up before heading to the Catlins, an exposed area at the bottom of the South Island, where they camped at various locations.
Beckenridge's car was found at the bottom of sheer cliffs near Curio Bay in the Catlins along with other things belonging to the duo including a tent, sleeping bags and empty fuel containers at a campsite - but the pair has never been heard from nor their bodies found.
The coroner's hearing continues.
