You'll often find deals online for pickup that are cheaper than buying at the duty free store - sometimes 10% - 20% off purchases.

As pointed out duty free isn't always a good deal. Champagne in particular is quite often cheaper to buy retail in Australia from big liquor stores like Dan Murphy's than it is from duty free in NZ or AU. You can even walk any of the big supermarket chains in NZ and by a bottle of Mumm champagne as an example and it's cheaper than duty free 52 weeks a year.