Re-entering Bulgaria

Bulgaria follows the same 90/180 days entry rules the Schengen area follows, but separate from it.

That means that if you have been in the country for 90 days in one block you need to be out of the country for 90 more days before you can come back in.
If your stay was less than 90 days or if your visits have been split in several shorter visits, the rule is that you can enter if you have been in the country for less than 90 days in the last 180 days.

It is a rolling period, so any day you have to be out of the country for at least 90 days in the last 180.

And that is not just whole days, it is any part of a day. If you leave a few minutes after midnight, those few minutes count as a whole day and if you enter a few minutes before midnight those also count as a whole day. And if you have to enter the country to change flights, so get a stamp in your passport, that also count as a visit.

On the other hand, there is not something like 'have used your days' and need a visa now. Either you are allowed in, with a visa replacing stamp or with a visa, or you are not allowed in. The only exception is if you need a stay longer than 90 days like for studies or work, for that you need a visa, but that is separate from the visitor stamp in your passport as Australian short term visitor.

I answered this question based on my knowledge of the rules, could not find a link for you right now.
I double checked on an IATA site.

According to this site from the start of 2021* an ETIAS authorisation will be required and Bulgaria will be part of the Schengen zone some time in the future but it is not yet now.
When Bulgaria enters the Schengen zone your stays there will be added to the stays in the rest of the Schengen zone and the total stay restricted to 90 days out of 180 days.
*According to the official site for ETIAS the system is not running yet and it might be not before well into 2022.