I basically agree with what you are saying.
But OP stated that BC manager on his own authority authorized gardener to maintain a private garden with no indication the owner was going to be billed for the service.
If we assume the BC manager has been given full authority to spend as he sees fit (unlikely in this case) it is misuse of BC funds to spend them on maintaining private property as that is not the purpose that they were collected for.
If you accept that BC Manager (or the OC) can spend BC funds to maintain what is effect private property where does it end. He could decide to refurbish one of the units entirely using BC funds (maybe one he owns). If this is the case in NSW the Strata Act is in worse shape than I imagined.
In QLD you cannot use BC funds to maintain anything except Common Property without a No Dissent vote and I naively assumed you had something similar in NSW.