A man who attacked a 10-month-old boy in his care and broke several of the child's bones has been jailed for three years.

Eugene Paul Harris, 32, originally blamed the child's injuries on other children "being rough", when charges were laid last year, but he pleaded guilty to amended counts last month.

In the High Court at Auckland today he was convicted of causing grievous bodily harm with intent to injure and injuring with intent to injure.

Though the victim was expected to recovery from the physical injuries the court heard of the emotional toll the violence had taken.

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Justice Pamela Andrews said the child, now aged 2, suffered from night terrors when first discharged from hospital.

"[Care givers] also report he has a propensity to lash out violently without warning or provocation," she said.

The baby was in the care of Harris and his partner in early February last year, when the abuse happened.

He was "happy and content and was rolling around using both hands" at his childcare centre on February 5, according to staff.

However, when he was returned to the centre two days later they noticed he was unusually unsettled.

"A childcare assistant believed [the baby] was in some sort of pain due to the fact that he would not stop crying. It was noticed [his] left arm was floppy and fell to his side," court documents said.

On February 7, childcare staff took the infant to an accident and emergency centre in Takanini, where his arm was x-rayed and found to be broken.

He was sent to the Kidz First children's hospital at Middlemore, where another x-ray found his shin bone was broken and a full skeletal x-ray uncovered other fractures.

Harris admitted to police that on Waitangi Day, when the attack happened, he had drunk pre-mixed bourbon and cola in his garage area from about 2pm.

Justice Andrews highlighted Harris's criminal history which saw an escalation in his violent offending since 2000 and said the trend "must be met with a firm condemnation".

However, she give him credit for a number of courses he completed while on remand and for his guilty plea.

Harris's lawyer Kelly-Ann Stoikoff said the time in custody had been sobering for him.

"He hopes to learn the skills necessary to raise his own children in a loving and safe environment. It is his greatest desire. He is hoping it's not too late and he is determined he'll make every effort," she said.