As lovetravellingoz points out, there may be a mandatory period for the vaccine to become effective, once administered. I think the current vaccines require two shots, either 21 or 28 days apart, respectively. So vaccination on arrival may not be feasible.
For non-citizens or non-PR, the requirement may be outright that you need to be vaccinated before you can board the plane.
For citizens and PR, or for those who cannot take the vaccine on medical grounds, or cannot procure a vaccine in their own country, there may be the requirement to have a permit indicating a place in quarantine on arrival, and possibly that the quarantine is pre-paid. New Zealand has something along these lines as a requirement for those needing visas.
Some things to consider for non-citizens and non-PR is the potential availability of the vaccine in Australia (it may be limited to those who are resident), and the cost. The latter could be quite expensive if there is a requirement for hotel quarantine, and no government subsidy for the vaccine itself.