In general, the Common Boundaries Act 1981 would apply and there have been several ACAT decisions applying the CB Act to strata situations. The fence marks the boundary between two lease areas, the common property and the unit area. In general, the result is that the unit owner and the Owners Corporation would bear half the cost each.
However, there could be exceptions such as if a resolution has been passed to make it fully the responsibility of the unit owner or the OC. If the site was built with uniform fences around each unit area, the OC might decide that it would be more efficient and result in better maintenance if the OC took on responsibility for maintenance of all the fences.
Where I am, fences around unit courtyards are discouraged and this was made explicit in a landscaping policy adopted by general meeting resolution. Fences were not part of the original construction. However, a unit owner will be given permission to erect a fence subject to certain agreed constraints on height, permissible styles, not enclosing any common property and a condition that the cost will be borne by the unit owner. This carries through to maintenance also being the unit owner’s responsibility in our situation.