Three police searches - carried out shortly after the Christchurch mosque attacks - were unlawful, the Independent Police Conduct Authority has found.
The searches, made without a warrant, happened during an operation codenamed Whakahaumanu to identify people of interest to national security following the attacks.
Police used powers under the Surveillance Act to enter homes and in some instances, seize firearms and firearms licenses.
The three unlawful searches in question were all in Canterbury, weeks after the massacre at the two mosques in Christchurch.
One person, only known as Mr X, became a person of interest after an anonymous call to Crimestoppers, alleging that he was involved in white supremacy, anti-Muslim hate speech and racist behaviour.
Mr X had a historical mental health issue and had previously been a member of a Facebook group that had an association with a website describing itself as "far right-wing".
Mr X had also posted comments indicating to Police that he might be opposed to the government's gun law reforms.
The second person, known as Mr Y, came to attention after the police received information that he had been posting far right material on Facebook.
Both had firearms seized from their properties.
Police visited the third person, known as Mr Z, after Facebook posts caused concern about his mental well-being - and officers seized a bong from his property.
What the IPCA said
The Independent Police Conduct Authority (IPCA) chair, Judge Colin Doherty, said the officers should have had a warrant in all three cases.
"Police are required to obtain a warrant to make a search with this time to do so but they may exercise powers under the Search and Surveillance Act without a warrant in situations of urgency," he said.
"We found that in the three cases here, there wasn't sufficient agency and they could have got a warrant."
The authority received 13 complaints about the searches.
Judge Doherty also said there had not been enough focus given to the searches, over what powers the police could or could not exercise.
The authority did not make any recommendations.
"Search and surveillance and the exercise of warrants is bread and butter for police, so every police officer ought to know what they should do. We noted that in these cases, apologies had been given to the subjects of the searches," he said.
Firearms owners 'vindicated'
The Council of Licensed Firearms Owners (COLFO), which helped some of the complainants, said its members felt vindicated by the IPCA decision.
COLFO chair, Michael Dowling, said the police made major mistakes.
"They are the enforcement power of our laws and so they need to act within the law whenever they act," Dowling said.
"In these events, it shows that perhaps the checks and safeties are not there, to make sure that they as enforcers of the law are not wound up in the emotion of an event."
Police say context important
Canterbury Police District Commander, Superintendent John Price fronted on Tuesday, admitting mistakes were made.
But he said it was important to know the context around the time the searches were carried out, as after the terror attacks, the country had never been in an environment like it before.
"At the time, they were in a very heightened operational context," Price said.
"They were trying to eliminate or mitigate the risk of further violence and ensure public safety. That was their intent and that was their principles they were applying."
Price said the police will learn from the findings.
"Look, we've always got to be open to lessons learned and laws are there to protect all people, and as such, [we] are absolutely always open to ensuring there are good checks and balances."
Police said they would be looking to work with the IPCA to find any solutions following the ruling.