There’s quite a lot about this on various areas of the internet, and what it comes down to is that, in the absence of any clearly defined by-law, it’s a grey area. And mostly it’s about how much it bothers your fellow owners (and how much your complaints irritate them).
The model by-laws for schemes created after 1996 and the standard by-laws for schemes created before 1996 include this: “An owner or occupier of a lot must not … use for his or her own purposes as a garden any portion of the common property.” The model by-laws add the rider “except with the prior written approval of the owners corporation.”
If you have a version of either by-law on your books, you can tell the strata manager to do something about it, unless they and the committee want to be dragged before NCAT on a section 232 complaint (requiring them to enforce by-laws).
Your next fall-back would be to claim they are obstructing common property and the most direct way to establish this would be for a delegation of disgruntled owners to go down and either sit in the chairs or try to move the pot plants and film the reaction of the owner.
But make no mistake, this is a land grab and property theft, by any other name. Let it slide and soon every other ground floor owner will be adding to their property and soon after that they’ll be claiming squatters rights.
Nip it in the bud, I say. And if your strata manager can’t see the wisdom of that, it’s because they’ve forgotten why they took on the job.
By the way, saying the law only applies if it’s a fixture is a convenient furphy. Cars aren’t fixed but you can ping them for being left on common property.