Australian nationals do not need a visa to visit New Zealand. Australians are deemed to hold resident status on arrival. This means that they can visit, live, and work in New Zealand without obtaining a visa.
If you are travelling to New Zealand from Australia and you are not Australian, whether you need a visa or eTANZ depends on your nationality and whether you hold permanent residence status in Australia, or not.
From October 1 st, 2019, Australian permanent residents are required to pre-register an Electronic Travel Authorization before travelling to New Zealand (but they are not charged the associated tourist levy).