What I would suggest is that you uninstall the Austpost module via the admin shipping options (don't worry, it won't delete any of your product data). Next, delete all of the files relating to the austpost module, then re-install it again,
This is probably going to be a LOT quicker and easier than trying to figure out exaclty which files have been overwritten or uploaded to the wrong place.
Before doing that though it may be wise to complete your aborted zencart update, because with only a partial update there is no telling what other hidden and unforeseen problems may occur.
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