Not as good as the free entry deal however $20 for an adult + child is reasonable if you are going up there. To get this offer, you need to show proof of New Zealand residency (e.g. drivers licence or bill).
Normal ticket price is $41 ($29 adult + $12 child) walk up price or $30.75 booked online ($21.75 adult + $9 child).
Kids 5 and under are free always.