Worst case scenario you collect your luggage at Auckland, go through Customs and turn right when you walk out the door to the check in counters and check in for your flight to Melbourne. As mentioned previously Auckland International terminal is not large (on a world scale).
Best case scenario you book your return flight to Auckland and your AKL-MEL flight with a carrier in the same “alliance” e.g. One World and at check in ask if they can check you all the way through to Melbourne. They will give you your boarding passes for both flights and you will be a transit passenger in Auckland and will not have to go through customs etc before boarding your flight to Melbourne.
I would recommend having a carry on with some clothes etc as airlines have been known to ‘misplace’ luggage.
Whichever you chose, have a great holiday!