The Act refers to the Owners Corp spending money on both Common Property and personal property (although I’m not sure what that means, exactly). Trying to think of a scenario where it would be reasonable for the Owners Corp to spend money on private property, the situation you have described fits the bill. Say owners were having trouble selling or letting their property and one of the reasons was that one neighbour wasn’t maintaining their garden (or was unable to do so). Wouldn’t it then be for the greater good to spend the money – everybody benefits, either directly or indirectly, rather than waiting for the process to grind through Fair Trading and the CTTT.
It may not be strictly legal (although I’m not convinced it isn’t) but it could be the right thing to do.
In any case, the easiest way to resolve this is to find out who made the decision and ask them why they made it. Keep us posted!