Australian politics has some similarity to the United States. Seats in their House of Representatives are distributed by the population of the state (with each state guaranteed a certain number of seats). The states have equal representation in their Senate (twelve senators each rather than two) and the election of senators are usually staggered (half from each state elected every three years).
This year, however, is an unusual election (scheduled for July 2). The existence of staggered terms plus the voting system for the Senate creates the possibility that the party that controls the House will not control the Senate. Unlike the U.S. which forces the parties to live with deadlock until the next regularly scheduled election, Australian law contains an out — the “double dissolution” election. Normally, only the House of Representatives dissolves — either through expiration of the term or through the Prime Minister requesting an early election. If the dissolution of the House occurs within the window for a half-Senate election (within the last year of a Senate term), the House and half-Senate election occurs at the same time (but the new Senators do not take office — except for the Senators from the two territories — until the old term expires).