The price advertised on the front of the AirNZ website includes tax (as Quokka will no doubt be pleased to hear), whereas the one being hyped by Best Flights and Flight Centre does not.

SYD-WLG return = $96 each way plus tax = $312. You may be wondering why return is so much less than 2x one ways (at $174 each) - this is because for the return fare, not only is the airport tax is payable at the airport rather than prepaid, it is also significantly less than what Sydney airport slug you with.

I think these fares fall into the K bucket - I'm about to book one so I'll let you know. Whatever the code, they are 'Smart Saver' fares and therefore not eligible for mileage accumulation, though I know various people have reported success in earning miles on UA for AirNZ Smart Saver fares.