I'll post this here because I am looking to save some cash in Sydney.

I have a few questions about the Sydney airport and customs. I am planning to return to the US from New Zealand. However, in order for me to use my frequent flyer miles, I will need to depart from the Sydney airport rather than New Zealand. Is it possible to enter and leave Australia on the same day with two different itineraries? In other words, do I need to process through Australian customs, pick up my bags, and go check in for my international flight back to the States? Or is there a way to do this within the international terminal without waiting in the customs line? Alternatively, do I need a visa for Australia if leave the airport for 1 night and catch my international flight the next day? Or do people stay in the airport overnight..is this allowed?

I'm trying to minimize my time in Australia because it's just a stopover and I don't want to spend a ton of cash.