A judge has blasted police over "significant" failures and ordered them to pay $175,000 to a man they accused of killing his baby son. 

Detectives went to extraordinary lengths in their investigation into Mr S, who cannot be named for legal reasons, after the death of the 11-month-old child five years ago. 

They bugged the apartment he shared with his wife, his car and phones for two months to try to find evidence of the man covering up the assault, but came up with nothing. 

The man was charged with manslaughter and a five-week trial at the High Court in Auckland in November 2019 ended in a hung jury. 

After a further three years in and out of court - which included being awarded costs and then police appealing that decision - the man has finally been awarded costs. 

Through their lawyer, Rachael Reed, QC, the parents of the boy said they "hope that no other family has to go through the ordeal they have suffered". 

Reed and barrister Katie Hogan, both counsel for Mr S on the costs application and appeal, added: "Through the trial, the dismissal of the charge, the costs application and then the Crown's unsuccessful appeal against the costs award, Mr S and his family have always been and remain loving and tight-knit despite the tragedy of their son's death. 

"The grief they suffered was compounded by the awful allegations they have had to endure. As the Court of Appeal said, the Crown's approach at the trial of Mr S for his son's manslaughter 'could easily have resulted in a significant miscarriage of justice in this case.' That miscarriage of justice would have played out in all of their lives." 

An active child who liked exploring, the baby was a strong crawler who could kneel and his grandparents had seen him pull himself up on to his feet using the couch. 

Just before 7pm on December 8, 2017, the father left his baby in the lounge while he ran the bath. His wife was at a work function and it was only the third or fourth time he had looked after his son alone. 

From the bathroom, the man heard a thud from the lounge. He saw his son crying on a soft mat on the floor. He picked him up and comforted him but within minutes his son lost consciousness. 

The father phoned the Plunket Helpline at 7 pm and told them his son had gone "floppy." 

When ambulance staff arrived 19 minutes later, they found the boy unresponsive. At Auckland's Starship hospital a scan revealed a 12-centimetre acute subdural haematoma caused by the tearing of a bridging vein. The pressure on the boy's brain intensified, reducing the supply of oxygen to his brain. 

The boy - the firstborn of the man and his wife - died eight days later. 

Almost a year later, the man was charged with manslaughter. 

The Crown alleged the man assaulted his son which caused a subdural haematoma found on his brain and caused his death. Prosecutor Henry Steele said the boy was either "a victim of an extremely rare accident or he was the victim of a fatal assault" - and those two possibilities were irreconcilable. 

Medical experts called by the Crown said the tears to the bridging vein in the boy's case were consistent with "blunt force impact trauma" but no weapon was ever found. 

Steele told the jury the Crown acknowledged the man was an attentive and loving father and the boy was wanted and loved. But something caused the man to snap, "to lose his rag" and assault his son. 

The Crown said it was not reasonably possible for the boy to have fallen in such a way to cause such a catastrophic injury, especially as he was still learning to stand. And the only evidence that the boy was able to pull himself up came from his maternal grandparents. 

The only possible explanation for the boy's injuries was an assault by his father, Steele said. 

But Reed said during the trial the boy's death was a "dreadful and highly unusual accident". 

And just because it was unlikely, it didn't mean it was impossible. 

The defence also gave evidence the boy could have had a condition known as benign enlargement of subarachnoid spaces (Bess) because his head circumference was larger than his body. 

Bess would have increased the baby's risk of injury, Reed told the jury in a closing statement. 

There were no other marks or injuries on the baby that would suggest he was the victim of abuse. There were no red flags at all, the court heard. 

Justice Cheryl Gwyn said it was appropriate to penalise the police. Photo / NZME 

The baby had no historical injuries, the man was an attentive father, and there was no history of family violence, no financial stresses or any reason for the man to hurt his son, the defence said. 

Post-mortem evidence from the defence showed there were no signs of the boy being slapped or punched. 

Over the course of the year, police bugged the man's apartment and devices, sought search warrants and investigated the couple's finances to find evidence of the assault but they didn't find a "shred", Reed said. And when weighed against all the evidence, it was inconceivable the man would hurt his son. 

Jay Jayamohan, a paediatric neurosurgeon at the John Radcliffe Hospital in Oxford, England, gave evidence for the defence. He said explained rare events do occur and used an analogy involving planes flying around the world every year. 

"If a plane is involved in a crash the response is not to say, 'well this plane can't have crashed because thousands of planes fly every day'," he said. 

The boy could have been the" very unlucky" baby who was involved in an accident that caused his fatal injuries, Jayamohan said. 

A judgment for costs obtained by the Weekend Herald shows a number of procedural failures including instances where the police and Crown provided relevant material only after defence counsel specifically requested it, and senior officers withheld redacted information. 

There were also delays in responding to requests for medical records, images and surveillance information and the Crown initially refused to disclose any information obtained from its own expert witnesses in response to defence expert reports. 

In the costs judgement, Justice Cheryl Gwyn said: "I find the failures were significant and without reasonable excuse. An award to penalise those responsible is appropriate. Based on my understanding of what happened and the explanations given by counsel for both sides I consider the appropriate party to make liable for those failures is the New Zealand police." 

An application for costs was based on the charge being dismissed, the prosecution not having sufficient evidence to support the conviction in the absence of contrary evidence, and the investigation into the alleged offence not being conducted in a reasonable or proper manner. 

The defence also emphasised the father's conduct throughout the lengthy investigation and trial was consistently exemplary, despite the pressure it placed on him and his family. 

This included co-operating with the police, medical professionals and social workers, even allowing police to attend his son's funeral and co-operating after the family discovered police had placed listening devices in his home, car and phones. 

The father sought costs of a quarter of a million dollars but Justice Gwyn determined that an award that reflected 70 per cent of that sum was appropriate under the Costs in Criminal Cases Act. He was awarded $30,000 against the police under the Criminal Procedure Act and $145,000 under the CCCA. The Court iof Appeal dismissed the police appeal.