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The Westminster system does not compel, either by law or by convention, a member to resign from his/her electorate and force a by-election upon resignation or loss of confidence as head of government. The decision is left up to the member, who may choose to do so after a satisfyingly long career or in an attempt to allow the party to present a fresh face without being encumbered by someone synonymous with past unpopular policies.
In Australia, very few Prime Ministers have resigned from parliament straight after having resigned as Prime Minister:
I'm not as familiar with the poliical history of other countries. Some examples of those who have not resigned from parliament straight after resigning as Prime Minister are John Major (UK), Margaret Thatcher (UK), Paul Martin (Canada) and Jenny Shipley (NZ).