The calculation is very important. From that linked Government release: "(i) five or more passengers, or five per cent or more of the total number of passengers on board the same flight (whichever is higher) tested positive for COVID-19 by arrival test."

That is, if there are 60 passengers on board the ban will be triggered not with 3 positive cases but with 5? And with 200 it will be trigerred not with 5 positive cases but with 10?

Of course with a larger passenger load the greater the chance one passenger will have non-compliant docs and the second 3-positive-cases rule will apply.