Please check if you need a visa or NZeTA to enter New Zealand. Most Australian citizens will not need either, most US citizens will need a NZeTA only. NZeTA applications can be submitted online without needing an invitation letter from the conference, they are usually processed within 72 hours.
Please make sure you apply via the official government webpage (here) as there are a number of websites offering “visa services” at highly inflated prices.
