Correct. for domestic fares a return offers no cost saving or other benefit besides having 1 tix with both flights.
Pricing to Australia is mixed, and all depends on whether there is a sale for either direction (and occasionally a Grab-a-Seat return). So a return From NZ could be cheaper than two one ways, unless the flight ex-Australia is on sale (and vv). Taxes ex-Australia are a significant contributor to the price.