At the federal (national) level, there can be no more than 30 ministers at one time and there is no set list of areas of ministerial responsibilities (portfolios). Some portfolios, such as Treasurer and Minister for Defence, usually stay the same. Other portfolios are added, removed, combined or split by each new government. For example, currently there is a Minister for Local Government and Territories and a Minister for Infrastructure and Transport. In the past there was only a Minister for Transport, Local Government and Territories (and no Minister for Infrastructure).
The Prime Minister decides which members of the government team should become ministers. The Prime Minister chooses experienced and knowledgeable government members to be ministers, and expects them to work together on behalf of the government. Whilst members of the government team may have views about who should become ministers, it is ultimately the Prime Minister who decides.
You can find a current list of ministers in the Australian Government here.
For information about state and territory ministers, visit the websites of the state and territory governments.