Australia–New Zealand relations

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.

Both countries are constitutional monarchies and Commonwealth realms – sharing the same person as the sovereign and independent head of state – with parliamentary democracies based on the Westminster system. Their only land border defines the western extent of the Ross Dependency and eastern extent of the Australian Antarctic Territory. They acknowledge two distinct maritime boundaries conclusively delimited by the Australia–New Zealand Maritime Treaty of 2004.

In 2017, a major poll showed that New Zealand was considered Australia's "best friend", a position previously held by the United States.