Australian electoral system
Australia has a system of parliamentary democracy, largely based on the Westminster system. Since 1920 Australia has used a preferential system of voting (PV), this system is also known as Instant Run-off voting (IRV) and the Single Transferable vote (STV). In Australia voting is compulsory for all Australian citizens over 18, except prisoners currently serving a 5 year term or more, it is also compulsory to be on the electoral roll.
 The Australian Parliament
The Australian parliament consists of two houses, the Senate (upper house) and the House of Representatives (HoR or lower house). The House of Representatives is similar to the British House of Commons and the Senate is similar to the British House of Lords. The government of the day and the Prime Minister come from the House of Representatives and the Senate represents the interests of the 6 states and two territories. All of Australia's States and territories use PV.
The House of Representatives is made up of MPs elected from 150 single member electorates all of which have a similar voter population. The Senate is also elected using Preferential Voting, but instead it consists of 76 MPs who are selected from 8 multiple member electorates representing the 6 states and 2 territories. The Senate uses the same preferential system as for the HoR but in conjunction with proportional representation. A senate term is twice that of the HoR so that 6 members are elected each normal election from each of the six states(approximately every three years) and both territory senators from the two territories face re-election each parliamentary term.
 Preferential voting in Australia
Although Australia has a system where unlike in the U.S. and Britain a vote for a minor party isn't wasted, the two party duopoly reigns supreme and had never been threatened. Minor parties are rarely if ever elected to the lower house (HoR), although strong indendent candidates much more frequently are. The noteworty parties in Australian politics today are the Australian Labor Party (ALP) which is similar in ideology to both the British Labour Party and the U.S. Democratic party; The Liberal Party are similar to the Tories in Britain and the Republican party in the U.S. and together with the National party, who represent rural and regional interests, they form a coalition to become the second major party. Leftwing minor parties inlcude the Greens and the Democrats and rightwing minor parties inlcude One Nation and Family First. All other minor parties are so insignificant that they won't be covered here, typically recieving less than 1% of the overall first preference vote.
 Theories for the continuation of the major party duopoly
Some argue that the reason that the major party duopoly is still apparent in Australia is because strategic voting still widely occurs in the PV system, that is that people frequently have to sacrifice their true preference in order to pick 'the lesser of two evils'. However very high votes for fringe candidates are often recorded without unintended repercussions and there is absolutely no evidence that strategic voting occurs on any scale in Australia. The reasons why the duopoly exists is for other reasons, partly because the minor parties provide a moderately effect on the extremes of the major parties. For instance the Australian Labor Party (ALP) wouldn't be so environmentally friendly if it weren't for the threat of voters deserting for the Australian Greens (Greens) and it's policies on the mandatory detention of refugees would probably be stronger as well.
On the other side of politics a Liberal party candidate for the seat of Ipswich near Brisbane, Pauline Hanson caused a stir by her widely percieved as racist comments about Aborigines and Asian immigration. As a consquence she was disowned by her party and went on to found Australia's most succesful minor party One Nation. However One Nation failed to do well partly because of constitutional failings in the party, but mostly because the John Winston Howard lead Liberal/National coalition appropriated some of her racist, inhumane and divise policies and succesfully implemented them in time to win the 2001 election.
 Reasons for the continuation of the major party duopoly
The reasons why minor parties are underepresented in the Australian HoR are:
a) They don't represent all that many people, not enough to get them over the line in PV.
b) They are not trusted enough to form an effective government, they are popularly thought of as idealistic and not practical.
c) They are often percieved as being 'one issue parties' with narrow agendas, such as 'the environment', 'lower fuel and beer excise' and 'nuclear disarmament'. Such Parties in Australia have such imaginative names as 'Citizen's Electoral Council', 'Fishing Party' and 'Family First'.
In conclusion Condorcet voting is far superior to PV or IRV, but PV is often unfairly sidelined as being 'little better than Plurality', when in fact it is a pretty good and solid half way house to complete democracy.
 Australian states and territories
All 6 Australian states and territories have compulsory voting and preferential voting, the ACT and Tasmania however use a proportional representation method of PV for the lower house called Hare-Clark. All states use PR-STV for their upper houses (senate equivilant) except Tasmania which I'm not sure of and Queensland which has abolished it's upper house. Minor parties do much better in these parliaments, often gaining seats in both houses and occaisonly holding the balance of power. This is largely due to smaller electorates and the structure of revenue raising and service delivery in the Australian federation. The Federal government does the vast bulk of revunue raising, income tax, company and cosumption taxes such as the GST, and the states are left to spend, once the money comes back to them from the federal government. Thus some people consider minor parties to be capable and wise spenders, but bad revenue raising and economic managers.
 Compulsory voting in Australia
Although in theory it is compulsory to vote in Australia and 95% of Australians do, it is not a strictly enforceable law. All that is really required by law is that the voter show up to a polling place between 7am and 6pm on polling day (always a saturday) and have their name ticked of the electoral roll. Once in the polling booth, due to the secret ballot, what you do is up to you, that is whether you choose to register a formal or an informal (spoilt) ballot paper. A fine for failing do so will set you back $50, unless you have a 'valid and reasonable excuse' as defined by the Australian Electoral Commission (AEC), what actually constitutes a valid excuse is of course a safely guarded secret.
But even if you fail to turn up on polling day and receive a letter afterwards for a fine unless you can provide a reasonable excuse, even then all you have to say is that 'I did vote'. They cannot prove that you didn't, maybe your name wasn't crossed off the electoral roll properly.
 Advantages of compulsory voting in Australia
Political parties don't need to spend their time and money convincing their own supporters to be bothered on the day, instead they can concentrate on winning voters who aren't traditional supporters, thus no party can afford to alienate a sizeable number of people, this also helps to break down the extremes of the two major parties.
It encourages responsible citizens, voting is no longer just a right in a democracy but a fundamental responsibility, on par with tax.
 Disadvantages of compulsory voting
I really cannot be bothered voting. It's only $50 a year for non-compliance
 Thoughts about Condorcet voting in Australia
People in Australia typically aren't aware of the existance of Condorcet voting in Australia.
 External Links
Notable sites critising PV include http://www.electionmethods.org/ note they have a lot of material on the disadvatages and un-democraticyness of IRV, but absolutely no Australian case study.
Notable Australian sites are:
http://www.southsearepublic.org/ constitutional blog
http://www.aec.gov.au electoral commission
http://www.aph.gov.au parliament house
http://www.johnquiggin.com political and economics blog
http://www.travelentrav.com humorous blog
http://www.mumble.com.au/ election and leadership analaysis and commentary
http://www.crikey.com.au/ major Australian news source
http://www.onlineopinion.com.au e-opinion site which covers the Australian constitution and voting systems
http://www.abc.com.au/elections/ The most comphrensive Australian election site run with the expertise of Antony Green
http://www.pollbludger.com/ elections website
http://psephos.adam-carr.net/ elections website
http://www.ozpolitics.info/ elections website
http://democratic.audit.anu.edu.au/ Democratic Audit