Details of how a man allegedly murdered his stepdaughter a year ago are being revealed in court for the first time

Aaron Archer is accused of killing two-year-old Ariah Roberts at Mangawhai while her mother was at the supermarket buying dinner.

The High Court in Auckland has heard Ariah had no injuries when her mother left the house.

Fifteen minutes later she was dead.

Her catastrophic head injuries have been compared to those caused by a serious car crash or falling at least two storeys.

Archer concedes he caused the child's death, but denies the murder charge, saying there was no intent, violence or abuse - and the injuries were accidental.