Is there a way I can carry the credit on my pre-pay account to another mobile provider?
No you cannot transfer your Prepay balance to another provider.
If required please see https://www.vodafone.co.nz/legal/terms-conditions/prepay/ (Section 17).
Right, so essentially if I have some balance left and I decide to port my number to another mobile network provider, you will retain the balance?
That is correct @MohitRavi
Best to use all credit before porting out.