I reckon you could argue it either way.
On one hand, you could cite legislation on boundaries to say it is 50:50.
On the other hand, you could argue that it is part of the structures of the unit that just happens to be at the edge of the unit area. The UTMA is clear that the owners of class B units are responsible to repair and maintain the structures of the unit.
Since the second provision from the UTMA is more specific than the more general legislation on boundaries, I’d think the second view trumps the first view.
I would be even further inclined to the second view if fences were optional and only put up by or at the request of the unit owner. Where I am, fences around unit areas are only present if the unit owner wanted it and paid for it and this being a unit owner responsibility to maintain has never been questioned.
It is possible for the owners corporation to take on what would otherwise be a class B unit owner’s responsibility (by special resolution, from possibly fallible memory). EG. the OC might resolve to take on responsible for all the fences or for painting the outsides of all the units.