Agree with everyone, you won't have trouble being understood. You will likely have trouble understanding though! Besides the NZ accent, many Kiwis speak softly which complicates the comprehension. Watch out for words that have a different meaning in NZ; the one that is the most embarrassing is "root". In NZ it's slang for, well, the F word in verb form. So rooting for your team or getting to the root of a problem will get you at the least some funny looks