A Flaxmere man has been found guilty today of killing his 2 year-old stepson.

The jury at the High Court in Napier have just returned their verdicts of guilty on both charges after six hours of deliberations.

Tamehana Huata, 19, pleaded not guilty to one charge of manslaughter and one of injuring with intent to injure Matiu Wereta at his Flaxmere home in October 2015.

Both the prosecution and defence agreed Matiu had died as a result of a serious head injury, but argued different cases as to how the head injury was caused.

The defence argued the multiple bruises, bite marks and fatal head injury was caused by Matiu tripping on a towel and accidentally falling into a brick wall.

However the jury's verdict means they agree with the Crown's case that argued Huata, who was 17 years-old at the time of the incident, found himself "out of his depth" looking after the boy and proceeded to lose the plot and inflict the serious head injury.

The crown also argued the unreasonability of the defence's scenario in which the numerous bruises, bite marks and the fatal head injury were all caused by the boy running into a brick wall and falling down two steps.

"Either that force has come from the defendant or that force has been generated by a 14 kilogram, two and a half year-old boy and he's done it to himself somehow, accidentally," he said.

In summary Mr Manning encouraged the jury to make their decisions in the context of the real world, alluding to the "narrow window" of just 34 minutes in which Matiu's injuries could have occurred.

Matiu's mother Errana Tiopira testified that when she left for work at 8.45am on October 12 2015 Matiu had only several scratches and marks from previously swimming at a pool and playing rugby with his cousins.

In the period between her departure and a call she received from Huata, telling her Matiu was unconscious, the boy's body had received blunt force trauma causing a subdural hematoma and numerous additional marks and scratches.

Mr Manning said the defence's argument that the entirety of Matiu's injuries were inflicted by accidentally falling into a brick wall stretched the bounds of credibility, and argued while Huata may be a good person even good people can lose their temper and make bad decisions.

"Parents can snap, event the best parents can snap," he said.

In her summary Justice Susan Thomas told the jury the death of a child is a tragedy, but that they should put aside any natural thoughts that someone should be held accountable and look at the evidence presented to the court only.

Huata will be sentenced on April 7.