Leaving and re-entering Australia (visa runs) on a 3 month multiple entry valid for 12 month visa Subclass 601 ETA?

I am a 16y/o British citizen and I will be travelling to Australia next week on a Subclass 601 ETA visa, I will be staying with my mothers boyfriend in his house in Brisbane and I’m welcome to stay as long as I like with him. My visa allows me to stay for 3 months at a time and is valid for 12 months on multiple... show more I am a 16y/o British citizen and I will be travelling to Australia next week on a Subclass 601 ETA visa, I will be staying with my mothers boyfriend in his house in Brisbane and I’m welcome to stay as long as I like with him. My visa allows me to stay for 3 months at a time and is valid for 12 months on multiple entries. Ideally I would like to stay as long as possible so will not be travelling home after 90 days (I have a 1 way ticket booked). Now here is where the uncertainty lies, as of now I have no onward travel booked and of course don’t plan on actually leaving back to England so I will need to dip out and back in to a near by country. I am unsure as to where would be the best to go for the visa run or if that even matters and for how long is necessary to stay out of the country. Can I just leave for 24 hours or should I go for longer? Can I go to New Zealand or would somewhere like Bali be a better idea? What about Fiji or Vanuatu? Does anyone have any experience with this at all? Also can I book a return ticket or would 2 one ways be better as I know I will most likely be asked to show proof of onward travel at the airport next week but I’m assuming if I show them a return that clearly shows my intentions of coming straight back to Aus they will not like it very much... is this correct? Will I be denied re-entry if I only leave for 2 days? Any help much appreciated as I need to book the flights or at least the flight out of Australia ASAP!
Thanks - Rio
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