Ministers are members of the federal government who have been allocated an area of responsibility for how Australia is run. This area of responsibility is known as a portfolio and some examples include health, finance, defence, and immigration. Ministers are part of the executive government which is responsible for developing government policy and putting government decisions into action.
There have been occasions when parliamentarians have been appointed as a minister without a specific portfolio. This is rare but examples include Stanley Bruce in 1932, Enid Lyons in 1949, and James Killen in 1982.
In the case of Stanley Bruce, Prime Minister Joseph Lyons appointed him to the role so he could represent the Prime Minister and Australian Government in London, without the limitations of a specific portfolio.
In the examples of Enid Lyons and James Killen, they were both given the responsibility of Vice-President of the Federal Executive Council. This is not a specific government portfolio but a role usually performed by a senior government minister who is part of the cabinet.