On an Iranian passport and having already obtained a New Zealand visa, my flight includes a stop in Australia. Do I have to obtain an Australian transit visa?
You need a transit visa as Iran is not in this list of nationalities able to transit without a visa.
Having completed the form on the Australian visa website, you do need a visa:
As shown in the image, this is free, allows you to stay for up to 72 hours and usually processed within 19 days. You can read more details and apply here.