If you're already in New Zealand and your mobile won't work, there may be a solution.
If you forgot to set up Global Roaming with your network provider before leaving home, or if your Prepay is out of credit, why not purchase a Vodafone New Zealand Prepay connection pack for NZ$29.95? They're available from any Vodafone store in New Zealand. Ask the people at the store to check that a New Zealand SIM card will work in your mobile.
The easiest thing to do is rent a temporary mobile from Vodarent. They can give you a choice of mobiles, at reasonable rates. Once you insert your home network SIM card, the temporary mobile takes on your usual number and all your calls will be billed to your home network account.
Some overseas networks prevent SIM cards from other networks working in their mobiles.
If you wish to use a Vodafone NZ SIM card in your existing mobile while you're roaming here in New Zealand, it pays to try a Vodafone NZ SIM card before purchasing one to ensure the SIM will work in your mobile.
For further information please check with your home network.