You need to be OUT at least as long as you were in. You may have a multiple-entry visa, but you are still limited to a max of 90 days from date of entry (provided you have sufficient funds for a stay of that long), and a max of 180 days in any 365-day period (not calendar yr, but 365 days, so no reset!).
No visa runs! You cannot go to, say, NZ for a few days & return to Australia. In fact, since Aus & NZ are so closely tied, you're not going to NZ & back to Australia.
What you needed was a subclass 600 visa - not ETA for subclass 601. They could grant you a maximum of 90, 180, or 365 days, and you would have to return home. Application fee for the 600 is AUD145, not 20, and allow 30 days for processing (they say 90% are processed within 23 days).
Get a return ticket for your own country, returning home within 90 days of arrival. NOTE: day of arrival is Day One; day of departure is counted as a separate day. Count carefully because this means you've got a max of 88 days in the country, plus day of arrival, plus day of departure. Overstay will get you barred from entry. You're not going to Indonesia for 90 days - too long for tourist visa - and you are a minor child & cannot travel alone! - and you would not be returning to your own country.