Any disease tourists brought with them, even if they only came overnight, could be communicated to the herd.
Of course it could be communicated to the herd. But herd immunity - at least in the most common understanding - is not about how likely a disease is brought to the herd. It is about how fast and how good a disease can spread in a herd once somebody from the herd has the disease.
None of them are checked for vaccinations, so none of them can be considered to contribute to herd immunity, and this is why they cannot be added to the population of the Australian herd.
There are also unvaccinated people within the herd. So why should not being check for vaccination automaticly put you out of the herd?
Either you are there at the time the disease is trying to spread --> you are part of the relevant herd, or you are not there --> you are not part of the relevant herd.
The concept doesn't care about who you are and where you are from. It only cares about how many of the indivduals being there are at that time are immune and how many are not.
One critical factor to herd immunity is that of vector. Immunized herd members are reckoned unable to transmit the disease. Therefore, adding a substantial attack vector of potentially infected persons to the 'edge' of the herd, as happens with tourists, directly impacts herd immunity negatively.
Granted. I was incorrect. Thanks for pointing it out.
This is why the tourists aren't part of the herd. Since they are only temporarily present, they do not contribute - other than as potential vectors of infection of the herd - to epidemic.
Thanks for your substantive reply!