A man's been sent to prison for causing serious injuries to a 6-week-old baby, after admitting he snapped over its crying.

Benjamin Lim was sentenced to six years and nine months' imprisonment at the Wellington District Court today.

The victim's mother read a statement to the court where she said she had nightmares of the baby being shaken and squeezed.

She said for 14 weeks she did not know if her son's neck would heal and that he had to wear a neck brace.

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Lim cried in the dock when the summary of facts were read out. The victim's mother's sobs were also heard in the court.

It involved details of up to 18 historical fractures that the baby had endured, including the right collarbone, ribs, a neck fracture and upper cervical spine.

When sentencing, Judge Denys Barry took into account Lim's remorse, that he expressed through a handwritten letter, and his early guilty pleas.

He said a psychological report found Lim had not coped with pressures of taking care of a baby which lead to "extreme" violence against the child.

Judge Barry said the upper spine injury was "very significant" and "potentially fatal and disabling".

He said it was good news that recovery seemed to be on the cards for the baby but that would be unclear until the future.

Judge Barry said Lim had effectively denied the child treatment as he was "the only one who knew he was effectively torturing the child for weeks."

"[The baby] should've been safe, nurtured and loved but was put through an unimaginable ordeal."

Lim was ordered to serve a minimum non-parole period of half the sentence.