The issue of what considerations are relevant/irrelevant must be determined in the light of the legislation that is being applied and the facts of the case.
Relevant legislation may list matters which the decision-maker must take into account in making decisions. If so, all those matters are relevant considerations. A failure to take those matters into account may lead to a decision being quashed.
In other cases, relevant legislation may provide a non-exhaustive list of some of the matters that the decision-maker ought to consider. While all the matters contained in the non-exhaustive list are relevant considerations, other relevant considerations may also arise from the facts of the case.
Finally, in some cases, relevant legislation will not list any factors which should be taken into account. In such cases, a decision-maker should consider the purpose of the power being exercised to determine what factors are relevant to its consideration. In these circumstances, where legislation does not set out explicitly what factors a decision-maker should consider, the Court may give the decision-maker leeway by not imposing a strict list of the issues which it considers the decision-maker should have taken into account.