Here is a quote from Principles and provisions of Memorandum No AG600000 which is a standard ref for responsibility in NSW.
b. Fencing, if they are shown as a thin, dotted or no line on the strata plan then they are treated in accordance with the Dividing Fences Act, 1991 and are treated as follows.
i. Divides two lots. Cost apportioned between the two lot owners.
ii. Divides one lot from common property. Cost apportioned between the lot owner and the owners corporation.
iii. Divides one lot from an adjoining property outside the scheme. Cost apportioned between owners corporation and the adjoining property owner.
This is a standard def for fencing responsibility and I believe it applies throughout Oz (def applies in QLD).