A lack of evidence has seen charges dropped against the Porirua parents of twin babies who had brain injuries consistent with "rapid" shaking.

At the Wellington District Court last week, Judge Peter Hobbs dismissed the charges against parents Kyle Henare Hotai and Tunushia Wikitoria Schuster - jointly accused on two counts of failing to provide proper parental care.

In a written decision, Judge Hobbs said the babies were born prematurely on January 6 last year at Wellington Hospital.

STUFF Tunushia Schuster (in 2010) who was accused of failing to provide proper parental care. (File photo)

When the babies returned to the hospital in April staff found both babies had injuries consistent with shaking.

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Despite evidence both babies suffered non-accidental injuries at least twice none of the injuries could be precisely dated or explained.

There was no evidence to show Hotai or Schuster inflicted the injuries, and on that basis the Crown chose not to charge the pair with injuring the twins. But – as those responsible for their care – they were charged with failing to protect them from injury.

After they were born, Schuster initially cared for the twins while Hotai worked fulltime. But in the months after they were born Schuster began having trouble coping, spending less and less time caring for them, according to Judge Hobbs.

The twins were left in the care of friends and extended family. During this time, it is alleged Schuster was using methamphetamine, the decision said.

In the early hours of April 15, 2017, Hotai woke to feed and change the twins, going back to bed with a baby either side of him. Schuster was away for the night.

Hotai woke later that morning to find one of the babies lying on the floor gasping for breath. He picked her up and shook her, and she went limp.

As he ran to a neighbour's house to call 111, he slipped on wet ground and dropped the baby.

He began CPR before an ambulance arrived.

The Crown has not charged Hotai for dropping the baby while trying to get help for her.

On admission to Wellington Hospital staff noticed she had a bulging fontanelle and bluish bruise on her jaw.

Staff discovered an old rib fracture and and scans found acute bleeding over the baby's brain, thought to be between a few hours and a few days old, while a blood test came back positive for severe bacterial infection.

There was also "chronic bleeding" across the surface of her brain believed to be weeks old, as well as more old rib fractures and retinal haemorrhages in both eyes.

"The medical opinion was that [her] pattern of injury was best explained by her being subject to more than one severe shaking incident."

The other baby was also examined and was also found to have broken ribs and brain bleeds sustained at different times.

The evidence was that both babies had suffered a "non-accidental injury" on at least two occasions each.

None of the injuries the twins were found to have sustained were apparent or obvious and Judge Hobbs said the charges were dropped because there was insufficient evidence the pair had failed in their legal duty to protect the twins.

The twins were no longer in their parents' care.

Detective Senior Sergeant Neil Holden of the Child Protection Team said nobody had been charged with assaulting either of the twins, and he urged anyone with information to contact police.

"These investigations are often complex, with dynamics that influence the enquiry and evidence available."

In child abuses cases police strived to work with all parties to obtain the best outcome, ultimately in the best interests of children, Holden said.

"Achieving this goal is aided by appropriate adult behaviour in situations where abuse can occur, and if abuse occurs, brave ownership of issues by adults so the wider situation can be addressed in a way connected to child protection. This may, or may not, involve prosecution."