The Australian Constitution has always envisaged the admission or establishment of new states. The fact is that no new states have been ever been admitted or established. The use of the word admitted probably refers to the possible admission of other British colonies. There seems to be no reason why other territories which were never British colonies could not become Australian states[i].
[The Haka could be Australian...if New Zealand were one or two Australian States]
From time to time there have been proposal to create new states most notably in New South Wales and Queensland. In New South Wales there have been proposals to establish new sates in the north, “New England”, the south, “Illawarra” and in the Riverina. There has also been a proposal to establish a new state in the North of Queensland, “Capricornia”.
There have been proposals to establish the Australian Capital Territory and the Northern Territory as states. In a referendum in 1998, the electors of the Northern Territory rejected a proposal for statehood which would have retained the name “Northern Territory”.
From time to time there have been proposals that New Zealand, Papua New Guinea, East Timor, and Fiji each be admitted as a state. Sometimes the suggestion has been that New Zealand be admitted as two states, one for the north and one for the South Island. There have also been proposals for an Aboriginal State.
Because it would not involve the dismemberment of an existing state , the most likely first new state would appear to be the Northern Territory.
[i] After reciting the agreement of the people to unite in one indissoluble Federal Commonwealth under the Crown, the Commonwealth of Australia Constitution Act enacted by the British Parliament states explains that it was also “ ... expedient to provide for the admission of other Australasian Colonies and possessions of the Queen”.
In addition section 6 of that Act says: "’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, as for the time being are parts of the Commonwealth, and such colonies or territories as may be admitted into or established by the Commonwealth as States; and each of such parts of the Commonwealth shall be called ‘a State’."