Reproduced from the Tourism New Zealand website:
What's the climate like?
New Zealand's seasons are the reverse of the Northern Hemisphere. This means that the warmest months are December, January and February, while the coldest are in June, July and August. Don't let cold months put you off - winters tend to be short and generally fairly mild.
That said, on any given day the weather and temperature can change unexpectedly and sometimes dramatically. You should be prepared for that, particularly if you are going hiking or doing other outdoor activities.
What clothes should I take?
Dress is informal and relaxed on most occasions. Smart casual clothes are acceptable at most restaurants and night-spots. Men are generally not expected to wear suits and ties, except in a few of the top formal bars and restaurants in major cities.
In summer a jacket and sweater should be included in your luggage should the weather turn cooler or you visit higher altitudes. You can expect some rain, so also include a light rainproof jacket or coat. If visiting between May and September, pack warm winter garments and layer your clothing.
All visitors to New Zealand must carry a passport that is valid for at least three months beyond the date you intend to leave the country.
Most visitors who intend to stay for less than three months do not require a visa. If you want to stay longer than three months, or your country of origin does not have a visa waiver agreement with New Zealand, then you will need to apply for a Visitor's Visa.
All major international credit cards can be used in New Zealand and Travellers Cheques are accepted at hotels, banks and some stores. If your credit card is encoded with a PIN number you will be able to withdraw cash from automatic teller machines (ATMs) situated at banks and shopping centres throughout the country.
Credit cards with "Smart Card" technology
Smart cards are payment cards that carry an embedded microchip allowing them to store encrypted, confidential information, and carry multiple applications from different industries alongside debit, credit, or prepaid payment applications. Please note these cards, which often have no magnetic strip, are generally accepted anywhere in New Zealand that has credit card facilities.