When the Australian Constitution was written in 1900 precautions were made to allow New Zealand to become a state at some point in the future. This is seen in the introductory section of the Constitution, in section 6, under Definitions.
“The States shall mean such of the colonies of New South Wales, New Zealand, Queensland, Tasmania, Victoria, Western Australia, and South Australia, including the northern territory of South Australia . . . and each of such parts of the Commonwealth shall be called a State.”
Despite not becoming a state New Zealand has enjoyed state like benefits including military, migratory and economic ties with Australia. There have also been propositions in more recent times to increase ties with New Zealand, bringing it closer to statehood, but due to fears of native rights, cultural differences, legislative differences (including the legalisation of same-sex marriage) and a strong sense of nationalism, it is extremely unlikely that such propositions will ever be enacted.