Approval of the OC is required under section 110 of the Act.
https://www.austlii.edu.au/au/legis/nsw/consol_act/ssma2015242/s110.html
If any of the work affects common property then you need to comply with section 111.
https://www.austlii.edu.au/au/legis/nsw/consol_act/ssma2015242/s111.html
A by-law is required because the owner must be responsible for the works, and any successors in title. Correspondence doesn’t do that.