A jury has found a Hastings man guilty of the manslaughter of his girlfriend's five-month-old son.

Trent Owen Ngaruhe Hapuku, 23, had denied killing Mikara Ranui Darius Reti, who died in January 2011 at a sleepout in suburban Flaxmere from severe internal bleeding after his liver was forced against his spine.

A jury of seven men and five women delivered their verdict in the High Court in Napier on Thursday morning after four hours of deliberations, Fairfax reported.

The Crown said Hapuku had been focused on a Playstation game and lashed out at the baby, the child of his girlfriend Jaime Reti, when he became irritated.

Crown medical experts argued Mikara died within "minutes to tens of minutes" after receiving a blow to his abdomen.