Words like ‘must’, ‘shall’ or ‘required’ normally impose a mandatory duty and remove discretion8, but as always, this is subject to contrary intention9. While there is no fixed formula, consider whether failing to comply would defeat the Act’s purpose. If so, the provision may be compulsory10.
In this case, a provision said ‘Parliament affirms as a principle that a minor shall only be detained as a … last resort’. The judge (at ) said this was simply aspirational. ‘Shall’ didn’t impose an obligation because, when read in context, it was conditioned by the word ‘principle’. There were also no consequences for failing to comply.
This case is from Episode 15 of interpretationNOW!
8 Ozone Manufacturing  SASC 91 (at ).
9 Episode 12 discussed the reverse – when ‘may’ means ‘must’.
Pearce & Geddes (at [11.1-11.3]). Project Blue Sky  HCA 28 (at