The idea is (or should be), you've come from a country with similar vaccination rates, so yeah you might have caught covid on the tube travelling to LHR. So you might spread covid to someone on the domestic flight to Darwin.
But, maybe you didn't catch covid in London. But someone in Sydney has covid, and gets on the same flight to Darwin, and now you've caught covid off them.
Point is, there will be covid around. We need to get out of this mentality that inside Australia = safe, outside Australia = bad. That's why we're going for 80% vaccination.