Hello slicendice
I think you need a common property rights by-law. To find out more about that, read Strata Schemes Management Act 2015, Part 7, Division 3, Sections 142-145. You need 75% of the owners present at a meeting to add a new by-law. If this dispute goes beyond mediation and into NCAT, NCAT will want to know if a common property rights by-law exists.
A common property rights by-law would explain who is responsible for maintaining any patch of common property. It would be clearer to everyone if you have a separate by-law for each patch of common property maintained by a different owner. If you only pass a resolution at a meeting without updating your by-laws, you’re not really covered against more complaints.