It s not really correct to say that under the Australian Constitution New Zealand has the right or even an invitation to join the Commonwealth of Australia A right would imply that NZ can join regardless of the Australian attitude to this and this simply isn t the case During the initial stages of planning for federation the possibility of NZ joining was a real one But by the time the Commonwealth of Australia Constitution Act was enacted by the Westminster Parliament in 1900 the possibility of NZ joining the Commonwealth of Australia as an original state was no longer realistic But since NZ was right next door and had a relatively small population it was still possible that NZ along with other nearby British possessions such as some of the islands in the Western Pacific would wish to join at a future date The preamble reflects this by stating Whereas the people of the current Australian states have agreed to unite and whereas it is expedient to provide for the admission into the Commonwealth of other Australasian Colonies and possessions of the Queen This just states the political reality of the time that in the eyes of the British Government it was expedient made sense for other nearby British colonies to jump on the bandwagon One large British colony is easier to administer not to mention is more capable of defending itself than several small ones NZ does however get explicit mention in one of the covering clauses The definition of the states in covering clause 6 includes reference to NZ If you read this carefully it does not say that NZ is a state of the Commonwealth or even that it has the right to become a state Rather it simply says that if it did join then it would be a state for the purposes of the Australian Constitution Since it didn t join at the time of Federation the only possibility of NZ joining is now under Chapter VI of the Constitution which deals with the admission or establishment of new states If NZ wanted to join it would along with anyone else who wanted to join have to do so according to the provisions of s 121 which states that The Parliament may admit to the Commonwealth or establish new States and may upon such admission or establishment make or impose such terms and conditions including the extent or representation in either House of Parliament as it thinks fit That s not a right It s entirely up to the Australian Parliament whether NZ or anyone else for that matter can join But again the chances of NZ joining now are of course almost non existent