Natural justice — in modern Canadian usage, procedural fairness — is the common law's minimum code for anyone deciding another's rights. Audi alteram partem: the person affected must know the case against them and have a fair chance to answer it. Nemo judex in causa sua: the decision-maker must be unbiased, in fact and in appearance.
The content flexes with the stakes: a licence renewal may require only notice and written comment; a professional-discipline hearing that could end a career approaches trial-like protection. Breach has a distinctive consequence — the decision is quashed regardless of whether it was substantively right, because a fair process is owed, not just a correct outcome.
The duty of fairness applies to virtually all administrative decisions affecting individuals (Nicholson, Cardinal), with its content calibrated by the Baker factors — and for judges themselves, the test is whether a reasonable, informed person would apprehend bias.