The Law Commission noted the anomalous situation of the IPCA in its 2012 Review of the OIA, The Public's Right to Know. It recommended that the Ministry of Justice conduct a review of all public agencies to determine what should and shouldn't be included (this of course has not occurred). It also provided a list of the Legislation Advisory Committee criteria on whether a body should be subject to the Act, as a guide to the process:
The IPCA is government-funded. Its chair and membership are appointed by the government. It is subject to the control of the Auditor-General. It serves a public purpose, described in its Act as "the investigation and resolution of complaints against the Police". This means it meets four of the five core criteria for application of the OIA. The Law Commission also recommended two more criteria: the degree of public ownership of the agency and "the potential for decisions of the agency to impact on members of the public". It meets both of those too.
What about its statutory independence and investigative and quasi-judicial functions? There are other independent crown entities with similar functions - for example, the Human Rights Commission, Privacy Commissioner, Commerce Commission, Criminal Cases Review Commission. Every single one of them is subject to the OIA. They protect investigative and quasi-judicial information in various ways (typically through a secrecy clause, which may be broad or narrow depending on when the agency was established, and which may or may not be justified), but fundamentally they are subject to the Act, allowing basic oversight. The IPCA is an anomaly in that regard, and at the least should be subject to the same regime.
If the IPCA was fully subject to the OIA, the "sensitive" information around investigations would clearly be subject to various withholding grounds: privacy and confidentiality for witness statements and details, and the maintenance of the law where there is still a chance of prosecution. And where there is no longer a chance of prosecution, there seems no case for secrecy. Its been established that police prosecution decisions are subject to the scrutiny of the OIA, even if they hide behind legal privilege, and there seems to be little justification to withhold similar information on resolved complaints. The only purpose that services is to continue the unaccountability of police, which is contrary to the purposes of both Acts.
Incidentally, this means that where the IPCA recommends prosecution and the police refuse, the details of the police's decision can be requested, and they will need to at least provide a summary with their reasons, allowing their decision-making to be scrutinised. I am surprised the media doesn't do this more often in controversial cases.