It's 6 months from the time of collection, basically for HIV (and possibley other diseases?) that may not show up at first, so they test again after 6 months just to make sure. If you have a donor in mind and want him to donate, you'll have to think about it and get him to donate at least 6 months before you want to start TTC. If you are ordering unknown donor sperm I think that would have already been through the quarantine period, so it shouldn't be an issue (unless Australia quarantine again because it comes from overseas?).

You'll definitely have to talk to your clinic about it though, they'll know the full details.