As a journalist, Hager was entitled to journalistic privilege, which meant he could protect his confidential hacking source, known as Rawshark.
After the High Court decision, former Green Party co-leader Metiria Turei asked the IPCA to investigate whether any police officers were guilty of misconduct; police policy on media-related search warrants; and decision-making process which led to the search warrant.
Hager also raised further questions about whether police overreacted in the case, and whether they breached his privacy by collecting information about his travel and finances despite not being suspected of committed a crime.
The IPCA said today that the police's errors in applying for the search warrant did not amount to misconduct. There was no evidence that they set out to deceive the judge by failing to mention that Hager was a journalist.
Police officers had little experience in dealing with issues of journalistic privilege, the IPCA's report found. Police policy on journalist privilege was also found to be deficient.
"The lead detective himself acknowledged he had never come across the issue of journalistic privilege before and did not know anything about it.
"It is therefore not surprising that he and others said that they simply acted on the legal advice they had received regarding the content of the search warrant application."
The IPCA said the application for the search warrant also failed to sufficiently address whether the officers had reasonable grounds to believe they would find relevant evidence at Hager's home. Its application was too broad when it came to digital evidence, and would have captured "a great deal of correspondence" which had nothing to do with Dirty Politics.
The IPCA also found that the police failed to fulfil their duty of candour when they seized private information about Hager from Air New Zealand, Paypal, NZ Customs and Jetstar.
"The authority … considers that police did not even come close to demonstrating that the officer making the application had reasonable grounds to believe that the police would find evidential material," Judge Colin Doherty said in his report.
He added: "These production orders cannot be characterised as anything other than a fishing expedition."
On Hager's complaint about police's response being disproportionate, the IPCA said it could not come to a conclusion because it did not have information about the competing demands for police resources at the time.
Green Party co-leader James Shaw said today that the report showed that police had overstepped the clear legal protections given to journalists.
"Police have questions to answer about whether journalists from now on can expect to receive better and more lawful treatment than Mr Hager did.
"Police need to assure all New Zealanders that they are making changes to policy and procedure, to ensure this kind of thing never happens again."
The mistakes made by police threatened vital media independence, Shaw said.
Dirty Politics, which was partly based on emails hacked from Slater's computer, detailed how former Prime Minister John Key's Government used blogs like Whaleoil to disseminate smears and scandals it did not want to be associated with.
Assistant Police Commissioner Bill Searle said today that the issues raised in the IPCA report had been "well traversed" in previous court hearings.
He said police had made revisions to the police manual in relation to privilege, and these changes had been approved by the IPCA.
