Aaron Archer has been found not guilty of the murder of his former girlfriend's two-year old daughter, but guilty of manslaughter.

Two-year-old Ariah Roberts sustained an unsurvivable head injury while in the care of Aaron Archer, 31, in their Mangawhai home on 22 August 2018.

Her mother, who has permanent name suppression, left the house for 15 minutes and returned to find her daughter unconscious and no longer breathing.

Ariah quickly died of a blunt force head injury and a post mortem later identified more than 20 bruises on her head.

2-year-old Ariah Roberts was killed in Mangawhai Heads in 2018. Aaron Archer was charged with her murder. Photo / Supplied

Three experts agreed her cause of death was a blunt force head injury but how the bruising to her head could be explained was disputed.

The Crown argued the nature, scale and pattern of bruising to the girl's head meant her injuries were non-accidental.

Archer declined to give a statement to the police - as is his right - but various witnesses said he told them he was swinging the girl around when she hit her head.

His defence team argued the sum of Ariah's bruises could be explained by an accidental impact, normal toddler tumbles and one incident when the girl was attacked by the pet cat and knocked her head on the coffee table.

During the course of the trial the court heard Ariah wasn't the first child her mother - who has permanent name suppression - has lost.

Ariah had been a twin but her brother was born premature and died.