Sheera, You should read through the entire “Division 2 Contributions by Owners” of the Strata Schemes Management Act (NSW) 2015.
A quick look at Strata Schemes Management Act (NSW) 2015 Section 81 suggests that you can change the levies at a meeting other than the AGM.
Paragraph (2) says “meeting”. There is no specific reference to “annual general meeting”.
If your strata manager or strata committee tells you otherwise, you can refer them to the Act.