Don't know about Australia, but the striker and latch on American 63's are different than 65's. Below, top two pix are my 63 convertible; bottom two are my 65 Barracuda. The supplemental latch doohickeys on the 63 (above and to the outside of the main latch) are unique to convertibles, both A and B bodies.
As for why your door won't shut, I can think of three possibilities:
1. Is your inside handle returning to the closed position (all the way forward)? On my 63, the springs on both door handles were so worn out they wouldn't return on their own - had to manually move them forward. If left in the open position, the doors would not close. (Fixed this problem by replacing the handle mechanisms with good used ones that had good springs.)
2. Is the striker aligned properly? There are three screws holding it to the door frame. May need to loosen them, move the striker, then tighten the screws. May have to do this more than once to get it properly aligned.
3. If neither of the above helps, then I'd have to think the latch itself is bad. I replaced both of mine with NOS from ebay. They show up now and then. Doors did close with the old latches, but it took a firmer push than it should have. With the new latches, a light touch closes the doors with a solid ka-chunk.
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