With a NZ ePassport and no checked luggage (and any items in your carry on that might arouse quarantine interest) you could clear customs in under 10 mins. I have on occasion picked up a rental car 15 minutes after stepping off an airplane (albeit in SYD).

Also, depending on which NZ airport you are departing from, you could clear the first part of the eGate process in NZ itself, thus proceeding directly to the actual eGate in Melbourne.

Ofcourse if your arrival is in any way delayed, you are SOL