A South Auckland man who fractured the skull of a four-month-old baby has appealed his sentence with his lawyer saying "there are skull fractures and there are skull fractures".

Toetu Salasa, 25, was sentenced to two years, two months in prison last year after he pleaded guilty to wounding with intent to injure and assault with intent to injure.

He injured the baby while it was being held by its mother, who he was attempting to punch in the face.

Salasa appeared in the High Court at Auckland today where his lawyer Panama Le'au'anae argued his client should have received a sentence of home detention.

"I accept the child suffered a skull fracture but there are skull fractures and there are skull fractures," he said.

Le'au'anae said medical notes showed the child suffered a small haematoma and a "minimally separated fracture".

The child was given paracetamol and spent four days in hospital under observation.

The little boy had since made a full recovery, Le'au'anae said.

Justice Susan Thomas questioned the wisdom of an appeal when Salasa had agreed to the police summary of facts and had pleaded guilty after being given a sentence indication that he would be sentenced with a starting point of three years.

Le'au'anae said he understood and wished to proceed with the appeal.

Prosecutor Natalie Small said the sentence the judge arrived at was appropriate.

The injury could only be described as "very serious violence on a very young child".

The assault took place on February 19 last year after Salasa was drinking at home while holding his 4-month old baby son.

His partner confronted him about how much he was drinking while he was holding the baby and she took the child from him.

They fought as he tried to wrestle the baby back from her and she punched him to make him stop pulling the child.

His partner put the baby's face against her shoulder protectively but Salasa began throwing "haymaker" punches at her head. The baby was hit in the right side of the head and received a fractured skull and a brain bleed.

The infant was rushed to Auckland's Starship children's hospital.

Salasa told police he punched his wife because he wanted to "repay" her and he must have hit the baby by accident.

Justice Thomas reserved her decision.

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 - Stuff