Schedule 2 Sect 31 applies, and specifically Clauses 2 & 3 where it states:
(2) A general meeting of an owners corporation (in this clause referred to as an “extraordinary general meeting” ), which is not an annual general meeting, may be convened by the executive committee at any time.
(3) The secretary of an executive committee or, in the secretary’s absence, any member of the executive committee must convene an extraordinary general meeting as soon as practicable after receiving a requisition for such a meeting signed by one or more persons entitled to vote in respect of one or more lots, the unit entitlement or the sum of the unit entitlements of which is at least one-quarter of the aggregate unit entitlements.
Part 3 of the Act prescribes how budgets and resultant levy contributions are calculated and raised, and whilst Sect 75 refers to these matters being determined at each Annual General Meeting, that’s merely because they’re nominally annual deposits, and it’s permissible to recalculate those in the circumstances that you’ve described and to resolve to amend the amounts at an Extraordinary General Meeting.