Ultimately, in our system of government, you have to rely on the courts. If matters are arbitrated, the Federal Court can decide whether withholding a redaction was unreasonable. The Federal Court can weigh the various interests, including national security, and so on.
What's in the public interest is very subjective. There are other mechanisms for feedback on government, such as the judicial inquiry unfolding before our eyes this week, or people litigating and taking the government to court. All those officers of Parliament—Auditor General, Privacy Commissioner, Commissioner of Lobbying, accessibility commissioner, Integrity Commissioner—get access to various forms of government information and provide valuable feedback on how government is serving Canadians.