It is charged per kilo over; if you want to be safe, purchase an amount which will meet requirements. If over the allowed amount of 20Kg, then will be NZD15 per kg

Pre-purchased baggage with booking is pretty cheap, for domestic NZ , 20Kg is $11 and if you think you might be a bit over it is only $20 for 25Kg or $25 for 30Kg