My understanding is that so long as you erect the fence within the boundary of your exclusive use area you do not need a further Owner’s Corporation approval.
Not strictly true. There are provisions in the act related to the appearance of the scheme being changed and the effect on common property of any attachment to it (for instance).
The simplest way to resolve these issues may be to create a by-law passing responsibility for the changes to the owners of the lot henceforth, and in the current rush to get the property on the market it may be wise to compromise so that the deal can be done. If the objecting neighbour contines to block approval, then you can seek orders at NCAT although that will take a long time. The easiest thing to do may be to ask then outright what they want and be prepared to compromise.