The Kahui twins had such horrific head injuries a senior doctor said they were worse than those seen in infants that had been in car crashes or run over.
Dr Patrick Kelly, a specialist paediatrician at Starship Hospital, was today giving evidence at the High Court murder trial of the twins' father, Chris Kahui.
He told the court bleeding and bruising to both babies' brains was caused by their brains smacking against their skulls at speed.
Kahui denies harming his then three-month old sons in June 2006 and is defending a double-murder trial that is now mid-way through its fourth week.
He says someone else - probably the babies mother Macsyna King - inflicted the fatal injuries.
Dr Kelly said the key factor in both babies' injuries was the trauma caused when their brains struck their skulls.
"The skull stops but the brain keeps moving of its own momemtum," he said.
He said shaking could well have occurred as well but he couldn't be certain.
Dr Kelly said the injuries suffered by the twins were the worst he had seen.
Crown prosecutor Simon Moore asked him what sort of surface could have caused the injuries and he replied the rails of the cot, the floor, wall or even the door of the nursery where they slept were hard enough to cause the babies head injuries, Dr Kelly said.
The impact wouldn't have caused external bleeding and would have only taken seconds.
Dr Kelly told the court if the babies had been conscious after the assault then they would have been crying - but otherwise it was likely no other noises would have been heard.
The trial continues.