I’m not in NSW so I am making a general point here.
I think successful management requires a little tolerance of ambiguity and judgement. If there is a general maintenance budget or a grounds budget line and the cost is small and the pot plant could be put against one of those and the EC would not be going over budget and the EC is confident that there would be general support for making the lobby a bit more pleasant with a few pot plants, then I think it should do it. At the next AGM the treasurer can write a plain english report explaining what the various lines in the financial statement actually mean and what they covered in the past year and what things might be done against those items in the coming year.
If the cost is not trivial in the grand scheme of things then put it to the next AGM.
For some things it is a matter of judgement. In your particular OC, will the EC be praised for having got on with some beautification rather than endless talking about it, or will they be condemned by a loud pedant who says that buying a few pot plants for inside rather than outside should not be counted against the admin fund line item for ‘gardening’?