Police believe one of the parents of 10-week-old twins found with multiple fractures to their limbs caused the injuries.
The parents, who cannot be named to protect the twins' identity, were sentenced to prison in the High Court in Auckland today on two charges of neglect.
However, they've never been charged with injuring the twins, who were found with dozens of fractures to their arms, legs, ribs and one of the twins' skulls in April 2012.
Senior Sergeant Adam Pyne said it could be very hard to prove who the perpetrator is in child abuse cases.
Mr Pyne said the police believed one or other of the parents caused the injuries, but did not have enough evidence to pinpoint which it was.
He said it was a challenging investigation but at least the parents had been held to account for neglect.
The father has been sentenced to four years and four months jail, while the mother was given a three-and-a-half year sentence, taking into account her ill health.
The parents had pleaded guilty to two charges of neglect of a child.
It was a meningitis scare that got the babies medical help.
Their mother took them to hospital after noticing spots on one of the babies and fearing they could have meningitis.
Medical staff found one of the babies had 13 fractures to their arms and legs and 11 broken ribs as well as a fractured skull.
The other baby suffered five fractures to their arms and legs and six broken ribs.
Justice Courtney said the couple's relationship had been marred by alcohol, cannabis use and violence.
Police had been called to the house twice and the father had been charged with an assault on the mother.
The judge said the babies would have been in considerable pain for days and that some of the injuries were two weeks' old.
Justice Courtney said both the defendants had other children and any parent would have known these babies were injured.
The judge said the children were at the most vulnerable time of their lives and completely reliant on their parents to get medical help if they needed it.
One of the babies also suffered from a heart murmur.
She said while the fractures would heal, the baby with the fractured skull could have ongoing effects and they may not be known for some time.
Crown prosecutor Fionnghuala Cuncannon said the police believed it was one parent or both but she said police couldn't prove who had done the crime beyond reasonable doubt.
The mother's lawyer, Frances Iggulden, said her client pleaded guilty after it was clear she would lose custody of her children.
She said the mother did not know who caused the injuries.
The father's lawyer, Malcolm English, said his client also didn't know.
In sentencing, Justice Courtney said Parliament had recently upgraded the penalty for child neglect from five to 10 years which sent a clear message to the courts.
She gave discounts for remorse and early guilty pleas.