He was discharged the next morning and was taken to an address where he died later that day.
Police said today that a post mortem has been completed but toxicology results have not been received so the boy's cause of death could not be confirmed.
Detective Sergeant Russell Richards said the investigation into the crash and the boy's death was ongoing.
He appealed for any witnesses who may have seen a white 2003 Suzuki Grand Vitara registration DDR570, in the western Kaitaia area last Sunday between 9pm and 11.30pm to contact police.
Anyone with information can contact Richards at the Kaitaia Police station on 09 408 6500.