Cook Strait can also be one of the world's roughest stretches of water. It's part of the westerly wind belt known as the Roaring Forties. As the only gap between the mountainous main islands of the country, the strait acts like a huge wind tunnel.

The Cook Strait, which separates New Zealand's North and South Islands, is widely regarded as one of the most dangerous and unpredictable bodies of water in the world. It is notorious for its "funnel effect," where powerful winds from the Tasman Sea and the South Pacific Ocean are squeezed through the narrow gap between the islands, creating extreme wind speeds even on otherwise calm days. The strait is also where two different tidal systems meet, causing erratic and powerful currents that can reach up to 7 knots. For the ferry crossing (Interislander or Bluebridge), conditions are usually "moderate" with swells of 1 to 2 meters. However, during frequent "Southerly" storms, swells can easily reach 4 to 6 meters (13 to 20 feet) or more, leading to significant ship motion and frequent seasickness among passengers. On rare occasions of extreme weather, the 3.5-hour ferry service is suspended entirely for safety. While the journey offers some of the most beautiful coastal scenery in the world, particularly through the Marlborough Sounds, travelers are always advised to check the "swell forecast" and carry motion sickness medication.