Kia Orana Sofeau,
DF alcohol is more expensive at Auckland airport stores. The cheapest option for purchasing DF in NZ is to order from a NZ online df store with collection at Auckland airport. Also, you can use loyalty cards in Auckland. Occassionally, stocks run low on Raro and some popular products aren't available until the next ship. This has hopefully been sorted out with 3 ships on the monthly between Auckland and Raro now.
As to the perfume purchase, yes you can wait until your departure to NZ and purchase it either at Rarotonga df shops in the departures lounge, or upon arrival into Auckland. NZ doesn't have a problem if passengers disembarking in Auckland carry df purchases made at Rarotonga airport within their carry on.
Having said that, the df shops at Auckland airport have a great range of perfumes and cosmetics airside in the departures & arrivals terminal. You also have the option of buying df upon departure, then collecting them upon arrival into Auckland. Both of the biggest df stores, DFS and JR, participate.