Norfolk Island is the only non-mainland Australian territory to have achieved self-governance. The Norfolk Island Act 1979, passed by the Parliament of Australia in 1979, is the Act under which the island was governed until the passing of the Norfolk Island Legislation Amendment Act 2015. The Australian government maintains authority on the island through an Administrator, currently Gary Hardgrave. From 1979 to 2015, a Legislative Assembly was elected by popular vote for terms of not more than three years, although legislation passed by the Australian Parliament could extend its laws to the territory at will, including the power to override any laws made by the assembly.

What is the only non-mainland Australian territory that is governed by itself?