For a 12-to-18-hour flight to New Zealand, you must prioritize breathable layers and compression. The "cabin climate" can swing from freezing to stuffy, so a base layer of moisture-wicking merino wool (a New Zealand staple!) is ideal. On top, wear a soft sweatshirt or a high-quality cardigan that you can easily remove. For bottoms, avoid jeans or anything with a stiff waistband; instead, choose "technical" joggers or high-end leggings that look stylish but offer maximum stretch. Compression socks are a non-negotiable peer-to-peer requirement to prevent Deep Vein Thrombosis (DVT) on such a long journey. Footwear should be easy-on, easy-off, like loafers or slip-on sneakers, as your feet will likely swell during the flight. Finally, bring a large, soft scarf that can double as a blanket. Since you'll be crossing multiple time zones, dressing in "athleisure" that looks decent enough for a hotel lobby but feels like pajamas will ensure you arrive in Auckland or Christchurch feeling as refreshed as possible after nearly a full day in the air.