A Palmerston North man has been jailed for a "despicable" assault on a 15-month-old child in his care that left the toddler with a broken thigh.

Brook Jonathan Trye, 25, was sent to prison for four years and two months for the assault on the boy, which he initially denied.

The boy belonged to his then-partner.

As the Palmerston North District Court heard yesterday, Trye's lies altered before he finally admitted on the eve of his trial in August last year to one charge of wounding with intent to cause grievous bodily harm.

Judge Gerard Lynch said that about 8am on July 27, 2009, Trye was trying to dress the boy in their lounge when the boy's mother heard screaming.

"When she tried to sit [the boy] down, he screamed and did not want to be touched. You told [the mother that] when you went to get [the boy] out of his cot, his legs were slipping out of the bars," the judge said.

At the time Trye said he found the boy crying, with his twisted leg dangling through the bars.

But he changed his mind, saying he was running down the corridor carrying the boy when he tripped over a cord and fell on him.

Trye said he would often run with the boy in his arms, although the boy's mother disputed this and said the cord was not lying across the floor.

Medical examinations found the broken thigh was likely caused by either a strong blow or twisting the leg until it cracked, Judge Lynch said.

In a victim impact statement read to the court, the mother said she feared the boy would blame her for what happened and was worried that she and her son would be in danger when Trye was released from prison.

"He's capable of deliberately breaking a leg of a 15-month-old infant he's been caring for; what will he be capable of in the future?"

The boy's mother said Trye had been controlling and had seen her son as a barrier to their relationship.

Judge Lynch said Trye had been diagnosed with an immature personality disorder, although this was not an excuse for his brutality.

The judge added that he found it difficult to gauge Trye's remorse, as he couldn't place any trust in what he said.

"A civilised society can be characterised by how it treats its young people.

"Your conduct was despicable," the judge said.

"An infant's wellbeing is in the hand of his or her carer. [The boy's] mother placed her absolute trust in you.

"No greater trust can be placed in a person."

Defence lawyer Andru Isac said there was nationwide disgust at violent acts against children, but Trye's offending was not long-running.

Instead his act was a "spontaneous one-off incident".