A Taranaki man assaulted his son after the youngster fearfully refused to take part in a physical fight with his step-brother.

A Taranaki father ordered his arguing sons to have a physical fight, and then assaulted one of the boys for not wanting to take part.

"Whoever wins is the strongest and I will not help if you get hurt," the 38-year-old man told his nine-year-old son and his step-son when the pair began to squabble while jumping on the trampoline.

According to a police summary of facts, the man intervened and insisted the pair have a fight to resolve their dispute.

But fearful of getting hurt by his bigger step-brother, the man's son refused to take part. He gave his dad, who cannot be named in order to protect the identity of the boy, the finger and ran off.

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This youngster was chased by his stepbrother and dad around the Waitara property.

The victim's sisters, who along with their step-sister were present during the May 2017 incident, became concerned for their brother and tried to prevent him from being caught.

But his dad got hold of him by the shirt collar and forced him inside the house.

The victim ended up on the floor in the lounge where his dad slapped him around the legs.

The boy began to scream and then wet his pants. The man then slapped his son across the face.

The victim's sisters attempted to console their brother but were told not to otherwise they would also be smacked.

When spoken to by police, the man said insisting the boys fight had been a joke and he then became mad when his son gave him the finger.

On Wednesday, he appeared before Judge Chris Sygrove at New Plymouth District Court where he was sentenced on one charge of assaulting a child.

Defence lawyer Turitea Bolstad sought a sentence of supervision for her client.

She said the defendant, who has previous convictions, was engaged in a parenting course ordered by a judge at an earlier appearance.

Judge Sygrove said he was sure the man had learnt a valuable lesson.

"Inviting children to fight to resolve disputes is noway to resolve disputes amongst children let alone adults," he said.

"And then to assault a nine-year-old is totally unacceptable."

But he praised the man for engaging in the course and sentenced him to nine months supervision.