SYDNEY (Reuters) - Australia will hold a general election on May 18, Prime Minister Scott Morrison announced on Thursday.

Here are some facts on how elections work in Australia:

** Australia does not have a set date for national elections but the maximum term between polls is three years. The election is called by the prime minister.

** There are two houses of parliament, with the government formed by the party or coalition with a majority in the lower chamber or House of Representatives.

** All seats in the House of Representatives will be up for election. Morrison’s Liberal-National coalition holds 74 of the 150 seats in the House of Representatives, the opposition Labor party has 69 seats, and seven are held by minor parties and independents.

** The new parliament will increase to 151 seats following a review of electoral boundaries and population changes. Victoria and the Australian Capital Territory each gained a seat, while South Australia lost one seat.

** The prime minister is chosen by the governing party from the House of Representatives.

** Voting is compulsory for about 16 million Australians, who must register when they turn 18. Those who don’t vote face a fine of A$20 ($14).

** The upper house Senate has 76 members - 12 from each of the six states and two from each of Australia’s two less-populous territories.

** State senators are elected for six-year terms, while territory senators are elected for three years. Half the Senate is up for election at each national poll, except in rare cases when parliament is deadlocked and the entire upper house is dissolved.

** Australia has a preferential voting system for elections to the lower house, with voters marking their ballot papers “1, 2, 3 ...” in order of preference.

** A candidate who gets more than 50 percent of the total first-preference votes wins the seat. If no candidate reaches that threshold, the one with the fewest votes is excluded and their votes are distributed to the person nominated as the second preference. This continues until one candidate passes the 50 percent threshold.

** Since 2010 there has been a high turnover of prime ministers in Australia, where the governing party can call a leadership vote without involving the electorate.

** No prime minister has served a full term in office since a John Howard-led coalition was voted out in 2007 after 11 years in power.

** Since 2010, Kevin Rudd (Labor), Julia Gillard (Labor), Rudd (for a second time), Tony Abbott (Liberal), Malcolm Turnbull (Liberal) and Scott Morrison (Liberal) have served as prime minister.