Almost 85 percent of New Zealand exports by value, and over 99 percent by volume, are carried by sea. In the case of imports, around 75 percent by value is carried by sea and the volume is also over 99 percent. Overseas lines Conference lines handle much of New Zealand's overseas shipping.
Conferences are associations between shipping companies to provide joint services on trade routes.
Increased competition in New Zealand's international trade over recent years has seen increasing participation by independent carriers. A ferry service across Cook Strait between Wellington and Picton is provided by three rail ferries operated by Tranz Rail's Interisland Line. The 'Arahura' and 'Aratere' carry passengers and vehicles. The 'Arahanga' carries freight with limited accommodation for passengers. Tranz Rail also operates a high-speed car and passenger ferry between Wellington and Picton over the summer season and for a limited winter season.