Generally this is a consequence of the broadcaster using the some series code and a different episode code. On Satellite PVR models in some cases this can lead multiple regions being recorded, the solution in this case is to delete the unwanted region from the channel list.
Which Humax box. The answer may be different depending on if it’s a satellite or terrestrial box. Generally it’s a broadcaster recording codes issue. Telling us which broadcaster would also help.
This is a UK based forum. We don’t have that model here. You would need to find a Oz based forum to get any sort of answers. Perhaps ask Humax Australia support.