Australia became one country the 1st January 1901. Until then it consisted of 6 colonies. There were several reasons for the colonies joining togather. They are cultural reason, institutions and defensive reason.
First of all the population of the colonies consisted of migrants from Britain and Ireland. That means that people have same language, same views of the world. Quite logically, the colonies' political and administrative institutions were very similar. This is the second reason.
Finally, it is defensive reason. The colonies were too small alone for an army as there was small population so there were few soldiers but the territory of each colony was large, on the contrary.
Thus, it was reasonable for the Australien colonnies to join togather as the people came from Britain and Ireland, their police, goverment and taxation were similar and they couldn't be defended from attack alone.