A judge has slammed the police investigation into a crash involving an off-duty officer saying there was a "regrettable but available perception" that it displayed bias in favour of their injured colleague.

Police were ordered to pay over $40,000 towards Auckland man Graham Hohepa Anderson's legal fees in March after Judge Gerard Winter found police were "negligent" in pursuing a case that never should have made it to trial.

He earlier had found Anderson not guilty of careless driving causing injury and said the prosecution had not excluded the "reasonable possibility" that the October 2013 crash was caused by off-duty police officer Mark Hansen, who was injured in the smash.

In his costs decision, Judge Winter stopped short of saying it was a case of police looking after their own, though there was a "perception" that "out of sympathy for a badly injured fellow policeman the police unreasonably pressed on to prosecute Mr Anderson".

Though he eventually ruled that there was insufficient evidence to find the prosecution was in bad faith, Judge Winter said there was an "available perception" that the police investigation was "tarnished with bias".

The judge criticised police for calling one of their own officers as an expert.

That witness, Sergeant Mile Tusevljak, did not know of his duties as an impartial expert witness and was "negligent" in preparing his reconstruction of the crash, the judge said.

The mistakes "thereby produced a completely flawed analysis of the accident's cause".

"Beyond this negligence the fact that the "victim" was an off-duty police officer provides a regrettable but available perception that the investigation about the incident and prosecution of Mr Anderson was tarnished with bias."

Allegations of police bias were also supported by police "unreasonably discounting" evidence of an on-coming truck driver who had a "grand stand view" of the accident but whose evidence was in favour of Anderson, the judge said.

In finding Anderson not guilty, Judge Winter relied on evidence that Hansen was tailgating two cars on Sandstone Rd, Whitford and, on a two-lane section of road, pulled into the right lane to overtake.

At an estimated 110kmh he was confronted by Anderson's truck in his lane and rather than stop he "took a risk and crossed the solid yellow line dividing the east and west-bound lanes", the judge said.

The crash occurred when Hansen cut sharply back onto the right side of the road, his car glancing off the side of Anderson's truck.

"Mr Anderson should never have been put to a defence of this charge," the judge said.

He ordered police to pay 75 per cent of costs and said Anderson's employer should pursue police in civil court for losses they suffered.