It's called "hidden city" ticketing. All the airlines hate it and you will be in violation of the Contract of Carriage.

Basically the rest of your trip (i.e., your flight(s) back) are cancelled (this wouldn't apply in this case since it is OW), your PNR gets flagged, and you won't get FF points for the flight. And you might get chased for the difference in prices.

If you buy a ticket to Sydney, they are under no obligation to get you to Auckland, so if something happens and they end up flying you direct to Sydney, or connect somewhere else, you are out of luck.

If you do this, you also can't check bags.