Trans-Tasman is an adjective used primarily to signify the relationship between Australia and New Zealand. The term refers to the Tasman Sea, which lies between the two countries. For example, trans-Tasman commerce refers to commerce between these two countries.A trans-Tasman flight is a flight between Australia and New Zealand. The Trans-Tasman Travel Arrangement is an agreement between Australia and New Zealand allowing their citizens free movement between the two countries. The Trans-Tasman Trophy is a Test cricket trophy.

Foreign relations between neighbouring countries Australia and New Zealand, also referred to as Trans-Tasman relations, are extremely close. Both countries share a British colonial heritage as antipodean Dominions and settler colonies, and both are part of the core Anglosphere. New Zealand sent representatives to the constitutional conventions which led to the uniting of the six Australian colonies but opted not to join. In the Boer War and in both world wars, New Zealand soldiers fought alongside Australian soldiers. In recent years the Closer Economic Relations free trade agreement and its predecessors have inspired ever-converging economic integration. Despite some shared similarities, the cultures of Australia and New Zealand also have their own sets of differences and there are sometimes differences of opinion which some have declared as symptomatic of sibling rivalry. This often centres upon sports and in commercio-economic tensions, such as those arising from the failure of Ansett Australia and those engendered by the formerly long-standing Australian ban on New Zealand apple imports.