I (US citizen) will be traveling from New Zealand to the United States via Australia on Qantas. There is a plane change and a 3-hour layover in Sydney. Do I need to go through Australian immigration? I will have an Australian visa as the first part of the trip is in Australia.
No, you do not clear customs in Australia in a NZ-Australia-US trip. You will spend the 3 hour layover in the Sydney transit lounge (they have a decent selection of dining and shopping options).
You will most likely still go through security for yourself and carry-on items before re-boarding the flight, but you will not need to clear customs, declare items, or claim and re-check your checked baggage.