Public Transport in New Zealand is not great..probably because they only have 6 million people across two islands both about the size of mainland Britain. Your best option would be to rent a car/campervan. The Kiwi Experience is also very useful for people travelling on a budget. The hop on, hop off system means you can stay as long as you want at each place as a new bus arrives everyday. For 18-30 yr olds this is the cheapest and most convenient way to travel..

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New Zealand & the Pacific Travel Forum
New Zealand & the Pacific Travel Talk