A secrecy clause is any clause in legislation which explicitly or implicitly exempts information from the coverage of the OIA. Such clauses allow information to be withheld under s18(c)(i) of the OIA as "contrary to an enactment".
Sometimes, this is done to protect important public interests: the Statistics Act protects your census records from disclosure, so that people won't be afraid to answer. Many agencies with inquiry powers can suppress information during an inquiry, and there's a standard clause temporarily exempting it from the OIA. Information which might identify whistleblowers can be refused, to protect their confidentiality and encourage people to come forward. But sometimes, the public interests at stake are less clear, the interests are already well-protected by the OIA itself, or the law goes far beyond what is necessary. For example, the longstanding confidentiality clause in the Tax Administration Act requires people dealing with tax information to keep it confidential, with criminal penalties for unauthorised disclosure, which protects personal privacy and is commensurate to its sensitivity. But it also allows the Commissioner to refuse to refuse to disclose information which would "adversely affect the integrity of the tax system or would prejudice the maintenance of the law", which are interests strongly protected by the OIA. In another example, the Crown Minerals Act has a clause forbidding them from disclosing any information relating to a mining permit. These clauses seem unjustifiable.
Which brings us to the current problem: the number of such clauses, and of poorly drafted or unjustifiable clauses, is increasing. Of the 91 secrecy clauses passed since 1987, 35 of them have been passed since 2010. 15 of those have been passed since 2017, and the government has another 7 in the queue. And many of them seem highly questionable. For example, the clause forbidding disclosure of information from organic products producers, or the many clauses (apparently driven by public sector dysfunction and mistrust of the Act) ostensibly allowing information to be shared betwen agencies, but as drafted forbidding its disclosure to the public. While there might in some cases be a public purpose there, the government is clearly not taking sufficient care with our right to know (as was made clear when they admitted their proposed secrecy clause for the Climate Change Commission was a drafting error).
What is to be done about this expansion of secrecy? In 1987, five years after the passage of the OIA, Geoffrey Palmer reviewed every secrecy clause which still remained on the books, and repealed those he thought were unjustified. Clearly, with the growth of secrecy over the past decade, we need to repeat the exercise. Every secrecy clause should be reviewed. Those which serve public purposes but which are overbroad should be narrowed. Those which do not serve such purposes, or where the interests protected are already protected by the OIA, should be repealed.