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If you don't have a passport, or if your current passport will expire less than 3 months after you plan to leave New Zealand, you will need to apply for a current passport. Plan to start the application or renewal process at least a few months before you plan to visit New Zealand. In the U.S., processing time for a new or renewed passport is usually about 4-6 weeks. Give yourself an additional 2-4 weeks to gather the necessary documents. Children and anyone else traveling with you will also need passports. When you apply for a New Zealand visa, you will need to submit a photo of yourself that meets very specific requirements for identification. While you can take the photo yourself, it's best to have it done by a professional. The photo should be 900-1200 pixels wide and 2250-3000 pixels high in a 3:4 aspect ratio. It must be less than 6 months old at the time you submit your visa application. Find a full list of photo requirements here: https://www.immigration.govt.nz/new-zealand-visas/apply-for-a-visa/tools-and-information/acceptable-photos. New Zealand requires that travelers submit a certificate of their health after medical examination if they plan to stay over 12 months. You can arrange for a general medical examination, or physical, with your doctor and ask them to send the results to New Zealand immigration through a program called eMedical. In some cases, you may need to provide a medical certificate even if you are staying for less than a year, or you may need to submit a chest x-ray as well. Find a full list of guidelines here: https://www.immigration.govt.nz/new-zealand-visas/apply-for-a-visa/tools-and-information/medical-info/evidence-you-in-good-health. One of the conditions of entering New Zealand is proof of good character. Your visa application may require a police certificate, which is called an Identity History Summary Check in the U.S. This is essentially a criminal background check, and can be requested directly from the FBI through their website. To make a request, go to https://www.edo.cjis.gov/#/. You will need to pay a fee and submit a set of fingerprints to obtain your Identity History Summary Check. If you have spent more than 5 years in another country since you were 17, you may need to request a police certificate from that country as well as your home country. You will have to show that you have enough savings to support yourself during your time in New Zealand. U.S. citizens are required to have at least NZD $1,000 set aside for each month they will be in New Zealand, or NZD$400 (USD$290) per month if your accommodations are already paid for. You may be able to get around this requirement if you can prove financial sponsorship by a New Zealand citizen. For instance, a friend or family member you will be visiting can agree to be your sponsor. To prove that you have the means to leave New Zealand before your visa expires, you will need to either already have a return ticket or be able to prove that you can afford one. Ticket prices can vary, so if you will buy your return ticket after you enter New Zealand, make sure you have enough to account for some fluctuation. If you do not have a return ticket, you may need to submit bank statements and other financial documents to prove you can afford one. Most regular health insurance plans don't cover you when you are out of the country, so it's a good idea to purchase travel insurance in case you need medical attention while you're abroad. It's recommended by the New Zealand immigration administration, and it will make your visa application look better if you can prove you're prepared to pay medical expenses. As part of the application process, you may have to formally agree that you will pay for any medical care you might need while in New Zealand.

Summary:
Obtain or renew your passport if necessary. Get a professional photo taken that meets immigration requirements. Get a medical certificate if you will stay in New Zealand more than 1 year. Obtain a police certificate or Identity History Summary Check. Plan to have NZD$1,000 (USD$725) for each month of travel. Buy a return ticket or save enough money to buy one. Get travel health insurance if you want to avoid high medical bills.