Where else could it possibly have come from? We have thousands of inbound travelers every week, law of probability says that we are going to get infections from this, it's just a matter of how often. Sure, total closure is problematic but maybe we could reduce that number significantly and in doing so, increase the time between outbreaks.
The first case was from Americold. They have it in Melbourne. While frozen contact was largely dismissed as unlikely, its known, and reported recently that it can stay very viable. While as Fred says, no matching genome is not 100% guaranteed you would have thought that you would get a clear match from the many infections we imported.
I think the incoming has dropped a lot, so that it matches those leaving MIQ
In any case, what we did in March worked, what we are doing now is working. Reduce the ability to find a host, that works.