"That failure contributed to the injury that Mr Legg suffered,'' said IPCA chairman Judge Sir David Carruthers.
Two IPCA representatives delivered a copy of the report to the Legg family today.
"Shane was upset.
"He's had a couple of sleepless nights knowing this report was coming. It's got him stressed out and reliving the whole ordeal again,'' Ray Legg said.
"Shane's life will never be the same and our lives will never be the same again. The officer could have pulled the wires apart or the other two cops on the other side could have helped. It was easy as that.''
He accepted Shane was no angel and deserved a kick in the backside for speeding, but did not deserve to have his neck broken while in police custody.
The watchdog has recommended police reconsider their decision not to take action against the officer, an acting sergeant, under the code of conduct.
It has also recommended amendments to the police manual to include the potential risks to the safety of handcuffed offenders.
Mr Legg, a disqualified driver with methamphetamine in his blood, was clocked doing 136km/h by police when he took off, abandoned his car and ran up a steep, wooded hill about 9pm on April 24 last year.
An officer and dog handler arrested him at the top of the hill.
Mr Legg, handcuffed with hands behind his back, was escorted to the bottom of the hill where he was told to climb over a metre-high fence.
In his attempt to climb it, Mr Legg fell and immediately cried out in pain.
He was then moved 30m-40m by two other officers and after complaining of a burning sensation in his back was laid on the ground while police called for an ambulance.
Mr Legg was taken to Whangarei Hospital and found to have a serious spinal cord injury.
In his complaint to the IPCA, he alleged his injuries were caused by police misconduct.
He claimed he landed on his head and immediately felt pins and needles throughout his body from the neck down and feared he had been paralysed.
The IPCA concluded that police actions during the arrest were reasonable and followed all relevant policies.
"However, the authority has found that while he did not intend to cause injury, the officer who instructed Mr Legg to jump over the fence failed in his duty of care because he did not consider possible risks and alternative courses of action,'' Sir David said.
"Although this was an error of judgement that proved to have serious consequences, the authority has found that this officer's actions were not so grossly negligent as to give rise to criminal liability.''
The officers who moved Mr Legg should have exercised more caution and checked on him before doing so, the IPCA said.
Northland Police today accepted the judgement, but noted the IPCA finding that the officer did not intend to cause injury.
"Unfortunately, an error of judgement by one of the officers, together with Mr Legg's own actions, has proved to have serious consequences,'' said district commander Superintendent Russell Le Prou.
"As the report points out, the circumstances of this arrest were very difficult and this officer had acted in good faith throughout the incident.
"Police take their duty of care very seriously. We arrest thousands of people every year and are often required to make split-second decisions in some trying situations. Our staff go out there to protect the community, not to cause harm. It is always upsetting when this sort of incident happens to people under their care.''
Mr Le Prou says Mr Legg has suffered very serious injuries that have changed his life.
"I will be meeting with the family to apologise and discuss our findings with them.''
