Why does Australia have 6 separate states?

Thanks for your question.

Australia has separate states because the 6 British colonies became the 6 states of a new democratic nation - Australia - through the process of Federation.

Australians celebrated Federation on 1 January 1901. But the history of this continent dates back much further. Aboriginal and Torres Strait Islander peoples have lived on the Australian continent for over 60,000 years. From the late 1700s, the British founded 6 colonies each with its own parliament, taxes and services, but all still under the lawmaking power of the British Parliament. 

In the 1880s and 1890s, many people began to identify themselves as Australians. They thought that the colonies might be stronger and more efficient if they worked together. Representatives from each colony met to create a set of rules which became the Australian Constitution. Between June 1899 and July 1900, the colonists voted in referendums to approve the new Constitution. Although groups such as women and Aboriginal and Torres Strait Islander peoples were mostly excluded, it was the first time in history that a group of people voted to create a new nation.

In July 1900, the British Parliament passed a law, the Commonwealth of Australia Constitution Act allowing the colonies to unite. Australia became a representative democracy, meaning Australians elect members of parliament to make laws and decisions on their behalf.  

The Northern Territory (NT) and the Australian Capital Territory (ACT) were created in 1911 under section 122 of the Australian Constitution. Both territories gained limited representation in the House of Representatives in 1922 (NT) and 1948 (ACT). It wasn't until 1975 that both territories were given Senate representation. Self government was approved later in 1978 (NT) and 1989 (ACT).

Australia is a representative democracy comprising 6 states and 2 territories.