In Kerikeri, you'll be on our roaming network so you'll need National Data.
You may be able to sign up to a Pay Monthly account if pay a cash bond more details here - http://www.2degreesmobile.co.nz/paymonthly/join.
Our $39 Pay Monthly plan as lchiu7 says offers 1GB of National Data, 2500 texts and 220 minutes. If you don't use the all of the first 120 minutes the remainder carry over to the next month.
Let us know if you have any more Q's.