You will need to apply for a visa before you travel into Australia, unless you are from New Zealand.
NZ citizens can apply for a free Special Category Visa (444) on arrival.
You will need a passport with a sufficient number of months left.
There are a number of visa options, depending on your passport. Dual citizens should check the requirements for each passport they hold.
The most likely visas you can apply for are the following. The links will take you to the government website where the conditions are described. Click on the [Details] button in the "Have an eligible passport" section to get a list of countries.
eVisitor (651) This is free and lasts up to 3 months. Mainly for European/Schengen passports
ETA - Electronic Travel Authority (601) This is also free but has a A$20 service fee to use the ETA app. It allows multiple 3 month stays in a 12 month period. This adds some Asian and American passports to the ones in the 651 list.
Visitor Visa (600) This costs A$190 and is valid for 3 -12 months. It is available for passport holders not eligible for 651 or 601 visas.
You can apply for these visas on-line . This link will show you which visa you can get.
If you are stopping over in other countries on your journey, please check visa requirements for each country. You may need a visa even if you are only in transit.