A round trip cruise is one that:

The time you spend on a round trip cruise counts as time in Australia for your visa, as long as:

Ask your cruise operator or travel agent if we consider your cruise to be a round trip cruise before embarking on your cruise.

If your cruise has ‘round trip cruise’ status, we will treat the time that a person spends on a round trip cruise as if they were still in Australia for the purposes of their visa.
A round trip cruise will not reset your stay period on your visa and you should check that you have sufficient time on your visa for the total time you intend to stay in Australia.
If the cruise does not have round trip cruise status, you should check that you have a visa that will allow you to return to Australia, especially if your temporary visa has a single travel facility.

Visiting Academic visa (subclass 419)