Hell Pizza are doing their standard CANDYMAN code for double sized pizzas (their standard size which is 12 inches) for Halloween. Although it's at the recent price of $16.66 instead of $13 like it used to be. Confirmed working right now. Standard delivery fees will apply if you do delivery.
According to comments, you might be able to use the AMEX shop small promotion for an additional $5 off via cashback if you haven't used your maximum.
Note that many Auckland locations seem to be open in the evening only today. I think it's a standard Monday thing rather than because of this promotion. And Hell tends to warn that their stores get very busy at peak times during this promotion.
Can get $11.66 for Amex holders with Shop Small Offer signed up after $5 cashback, provided the Hell Pizza store supports it and if you haven't used up the 10x cashback quota :)