"This is because there is an ongoing investigation into the overall circumstances of this matter that has not yet been concluded," Northland police spokeswoman Sarah Kennett wrote.
She said the documents might be released once the inquiry was finished, "although there may well be other withholding grounds that police will need to consider".
Detective Inspector Stuart Allsopp-Smith, based in Auckland, led the criminal investigation into Blowers, a 21-year police veteran, and will also conduct the internal review. He said the focus needed to be on the prosecution of Blowers ahead of wider issues to be examined. "Any lessons to be learned will be identified."
The downfall of Blowers started with his affair with an informant. As far back as 2002, Northland police bosses had told him to cease all contact with the woman, whose identity is suppressed.
Yet when one of the detectives in Blowers' organised crime squad raised fresh concerns about his boss' contact with the woman 10 years later, he was himself investigated.
Andrew Glendinning became suspicious of Blowers' behaviour and tailed him on visits to the home of a woman before handing a dossier - which included covert photographs - to senior Northland management.
But the actions of Mr Glendinning, not Blowers, were investigated first. He was removed from the squad, placed under strict supervision and subjected to an internal code-of-conduct inquiry. This centred on his use of the National Intelligence Application computer system, meant for official police business only.
After several weeks, he was cleared of any breaches and focus switched to Blowers, who became the subject of an internal inquiry, later escalated to a criminal investigation.
The wider internal inquiry review includes all steps taken by police management in Northland since the affair was exposed in 2002.
This is likely to include the decisions of Detective Inspector Karyn Malthus, who was the head of the CIB when the whistleblower raised fresh concerns. She went on to lead the "Roast Busters" investigation as the most senior detective in Auckland and was this year promoted to the rank of superintendent as the district commander for Tasman.
Blowers had maintained his innocence since his arrest in 2013 but made a shock admission during his High Court trial in October when he pleaded guilty to supplying methamphetamine over 12 months and a charge of stealing the drugs.
He took 34g of the Class A drug from the exhibit locker at Whangarei, substituting salt to disguise the theft.
Justice Geoffrey Venning emphasised the "high level of hypocrisy" of the crimes and said Blowers' actions endangered "the trust that members of the community properly have in our police force".
Timeline
2002: Detective Sergeant Michael Blowers was told by superiors to cease contact with an informant because of an "inappropriate and intimate relationship".
2011: Police raided a motel and seized 58g of methamphetamine. Blowers took the drugs from the exhibit room, removed 34g and replaced it with salt.
2012: A detective in Blowers' squad became suspicious of his boss and conducted a private investigation. When he raised concerns with management, he was put under supervision, before attention eventually switched to Blowers.
2013: Blowers was arrested and pleaded not guilty to methamphetamine charges.
2014: Blowers made a late guilty plea at trial. He was sentenced to four years and nine months in prison.
2015: Police launch an inquiry into the "wider issues".
