Costs
The strength of the AUSTRALIAN DOLLAR (AUD$) will make perhaps the biggest difference to foreign visitors. That said, Australia, is by regional and global standards, expensive.
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At the budget end (staying in hostels, rarely eating out, walking and visiting mostly free sights), you might be able to get away with about $8o to $100 per person/day.
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A more average budget for most visitors (staying at standard hotels or B&Bs, eating out once a day, going to the full range of sights and taking public transport), would be at least double that, at $160 to $250 per person/day.
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A daily budget of over $350 per person/day buys a pretty high-end experience, with smart hotels, guided tours and experiences, meals at a full range of restaurants and the use of taxis or Ubers to get around town.
Note that traveling solo will probably raise any of these per-person-costs by at least a third for all except budget travellers.