A convention is an accepted practice, an "unwritten rule", that has evolved by common agreement and usage over time.
The convention may apply to the operation of the Constitution, the Parliament, or the Executive Government.
The Westminster system is grounded in the idea that Parliament is supreme, that the government is drawn from the Parliament and is accountable to the Parliament, that legislation is approved on its merits by the elected members, and that the Parliament scrutinises the actions of the government.
In practice, the party discipline exercised by the major political parties has meant that much of what happens in Parliament is predictable and orchestrated. For examples, governments are rarely decided by votes in Parliament. The last time a government was defeated in the House of Representatives was in 1941. Similarly, backbench members on both sides are expected to be loyal "foot soldiers" who support their party in debate and votes on the floor of the Parliament. Critics claim that this has devalued the role played by Parliament and allowed the key personnel in the political parties to determine the course of parliamentary proceedings, as well as allowing government ministers to avoid scrutiny.
On the other hand, political parties have also contributed to the success of the Westminster system by promoting political stability. It would be impractical and unstable to have all members of parliament acting completely independently at all times. There would be little certainty about the survival of the government or the fate of legislation if this were the case.
It must also be remembered that political parties provide a means for alternative political views to be expressed in Parliament in a coherent manner. This can be seen through the existence of minor parties, such as the Australian Democrats, or One Nation, particularly in the Senate.
It must not be forgotten that political parties do not always control their members. Some members have been known to cross the floor to vote with the other side, particularly on matters of conscience. However, the power wielded by the parties means that members who behave in this manner may find their political careers in jeopardy, either through lack of promotion, or loss of pre-selection.
On balance, there is no doubt that organised and disciplined political parties have radically affected the way the Westminster system operates. Overall, they have contributed to a shift in power from the Parliament to the Executive arm.