It is basically how each leader came into power. For example, in 1975 Gough Whitlam was Prime Minister and the opposition held control of the senate. The senate blocked supply, so no money was able to be approved for government spending. The Governor-General removed Whitlam and appointed Malcolm Fraser as leader of the nation. He then called a Double Dissolution, so every standing member of both houses had to face a sudden election.
More information can be found on the internet, this issue in particular is available in more depth on Wikipedia: 1975 Australian constitutional crisis
Australian Election Pokemon
Topics