Ok, thanks for the clarification. Are those choices in the shipping address on the checkout page?
eCart will store the selected shipping country in a a session variable named:
eCartCheckoutForm_shipping_country
you can use this session variable as a trigger. if the value for Nort New Zeland is "NNZ" and South New Zealand is "SNZ":
you can add triggers to the shipping rules:
Based on Session Variable value:
If the session Variable "eCartCheckoutForm_shipping_country" has a value of "NNZ"
For the south island:
Based on Session Variable value:
If the session Variable "eCartCheckoutForm_shipping_country" has a value of "SNZ"