A South Auckland man who fractured the skull of a four-month-old baby has been jailed for more than two years.

Toetu Salasa, 25, appeared in the Manukau District Court today for sentencing after pleading 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, the court heard.

Judge Jane Lovell-Smith said that on February 19 last year, 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 judge said.

The 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.

Judge Lovell-Smith said it was an attack that injured "a completely defenceless child while being held by its mother".

Though Salasa had only been introduced to alcohol since moving to New Zealand, "your complete loss of self control and the dreadful violence that occurred is not an excuse", the judge said.

She said home detention was not appropriate and sentenced him to two years and two months jail.

Salasa will probably be deported to Samoa once he is released from prison.

The judge said Salasa's partner had taken part in restorative justice meetings with her husband and wished to reconcile with him.