A couple whose twin babies suffered more than 36 fractures while in their care have been jailed for a total of nearly eight years.

Despite the severe injuries, the defendants - aged in their 30s - faced only two counts of child neglect rather than violence charges because police could not prove who caused the harm.

"It must have been one or both of the defendants," prosecutor Fionnghuala [EDS: CORRECT] Cuncannon said.

Justice Patricia Courtney in the High Court at Auckland today granted them name suppression but stressed the purpose was to protect the children rather than their parents.

"There were many, many separate injuries which means the children would have been in pain for days on end," she said.

"It's not credible to think the level of pain and suffering could have gone unnoticed by two adults."

The violent acts likely took place over a two-week period and resulted in 26 fractures to one child - including a fractured skull - and 11 to the other, Ms Cuncannon said.

The kids were about 10-weeks-old at the time and had been born prematurely.

The offending was further aggravated by the fact one of the babies had a heart condition, the Crown said.

It was only when the mother noticed red spots and thought one child might have meningitis that they sought medical attention.

"There can be no doubt the babies would have been in substantial pain and the bruising on their bodies was evident," the judge said.

"It's important not to lose sight of the scale of the offending when we're dealing with two tiny babies."

Ms Cuncannon said there would be long-term effects to the children, who were taken from the care of their parents after the offending was revealed.

A report from a social worker said they were doing better in their new environment but exhibited some behavioural issues such as tantrums.

The long-term effects were difficult to predict, the judge said.

Malcolm English, defence counsel for the father of the twins, emphasised his client could not be sentenced on the basis that he had caused the plethora of injuries to the babies.

It was accepted the man had a previous conviction for assaulting his co-defendant some months before she gave birth, but Mr English said it was not relevant to the case before the court.

Justice Courtney said the pair's relationship was characterised by drug and alcohol use and violence and police had been called out to the house twice in the months before the child neglect was discovered.

The man received sentence discounts for his remorse, guilty pleas, his efforts at rehabilitation and his previous good record as a parent and was jailed for four years, four months.

His former partner got credit for her ill health, remorse, previous clean record and guilty pleas. She received a three-and-a-half-year prison term.