Composition: 76 members. 12 from each of the six states and two from the two territories, elected as follows:
*Under the normal electoral cycle: Only 6 of the senators from each state are elected at each "half-election", serving for two three year terms. All the Senators from the territories are elected in a normal "half-election". The terms are fixed; however the timing of the elections is sufficiently flexible that most elections for the House of Representatives are timed to overlap with a Senate election.
*In a "double dissolution": As part of the conflict resolution mechanism (which I'll come to below), the entire Senate is dissolved and put up for re-election, alongside the House of Representatives.
Election is by Single Transferable Vote. The number of candidates can immense - at the last election there were 78 standing in New South Wales. Australia has compulsory voting and it is compulsory for a voter to deploy all their preferences. A voter has two ways to cast a vote. They either rank all candidates in order of preference (yes even if there are 78!), and remember they've also just preferenced a House vote. Now you can understand the value of "How To Vote" cards recommending an order of preference. Or they can formally opt to have their vote redistributed in accordance with an order lodged by a party - for this they just mark a box "above the line". It's controversial in some quarters, especially due to horse trading, but frankly it's just a quicker way to vote the way a party suggests than copying out the How To Vote card.
The other point of note is how mid-term vacancies are filled. Until 1977 a state legislature would just appoint a replacement, with an unwritten convention that they appoint the nominee of the ex Senator's party. In 1975 (and we'll be hearing a lot about 1975 as we go) this convention was breached twice (and altered the balance of power in the Senate). A 1977 constitutional amendment now requires the new Senator to be from the same party, but doesn't require them to be the party's nominee. And some state legislatures have just declined to appoint a new Senator, leaving the seat vacant.
Fairness of representation: As I've said before, it's not always clear exactly what is the basis of representation in upper houses so the term "malapportionment" is best avoided. Depending upon how you look at it, the Senate either has a very fair representation for the states, with all having the same number of Senators, or a very bad voter-representative ratio with Senators from Tasmania representing an electorate many times smaller than those from New South Wales.
In a federal system a bicameral legislature with one chamber based on equal representation for the people and the other on equal representation for the states has some merits. However as the Senate is directly elected (as opposed to, say, the Bundesrat in Germany, which I'll look at in another post) and partisan it's questionable as to whether or not Senators are always acting in the interests of their state or their party.
The territories are the weak point in this as they don't get equal representation (but have even smaller populations). And when the Northern Territory was offered statehood in a referendum in 1998 it was only offered 3 Senators, rather than 12. What would be patently unfair would be for the original states to have more Senators just because they were the first in the federation.
Powers and conventions: The Senate is extremely powerful by the standards of second chambers and it's not for nothing that the term "Washminster System" is sometimes used in place of Westminster System. The Senate has near equal powers to the House of Representatives. Budgets ("Supply") and Appropriation Bills must originate in the House and cannot be amended by the Senate, but otherwise the Senate is in quite a strong position. It can reject Bills outright, unless the government initiates the double dissolution process (again, more on this later).
One particular point of note is that the Senate has the power to reject Supply. This is one of the most contentious matters. In practice the Senate has almost always not exercised the powers, and for most of the last 32 years the Senate has either been controlled by the government of the day or had the balance of power held by a party with an explicit stance against exercising the power to block Supply.
But in 1975, the one year in which the Senate did block Supply, constitutional chaos ensued. Strictly speaking the Senate didn't vote down Supply but rather passed deferral motions and set on a confrontational course with the House of Representatives and the government, who held office by virtue of their majority there.
Conflict resolution: A formal mechanism is built in, with the presumption that the House will prevail if it's the will of the people. If the Senate repeatedly refuses to pass a Bill, the government can advise the Governor General to call a double dissolution in which both the House and Senate face re-election by the people. (In practice a government will store up several bills and take them all to the people in one double dissolution.) Then a special joint session of House and Senate is called and the final fate of the Bills is settled there. The House generally has about double the number of members the Senate has, so is expected to prevail in a joint sitting. (In practice only one joint sitting has been held, in 1974, with the other double dissolutions leading to either the incumbent government losing power, gaining a majority in the Senate or not having the numbers to win a joint sitting.)
The situation in 1975 was more complicated, with both the Senate and government of Gough Whitlam standing firm. Whitlam asserted the Senate were challenging the more legitimate House and should pass the Budget, the Liberal-Country controlled Senate demanded a new election in the hope of gaining power for themselves. In the end the Governor General dismissed Whitlam and appointed the Liberal leader, Malcolm Fraser, as Prime Minister. This was the most controversial event in the political history of Australia and ever since there has been no real consensus on a way to resolve the issues involved.
Protector against the tyranny of the majority vs bastion against democracy: A tricky one this. In recent years the Senate has generally had no one party majority (although the last election proved a narrow exception) and so governments have not had free reign. However when an opposition party can secure a majority - and as well as at an election this can happen if state legislatures refuse to appoint replacement senators for government vacancies - then it can set on a course on confrontation.
Since the double dissolution takes the entire Parliament, house and both halves of the Senate et al, to the electorate it is in theory hard for the Senate to frustrate the will of the people long term. However if the elected government doesn't have a majority in the new Senate or the numbers to win in a joint sitting then it can't get its way. This should be because of ticket splitting and voter caution, though given the different voter-representative ratios in the Senate it is entirely possible for every voter to vote the same way for both chambers and still return different majorities.
And of course 1975 shows the dangers when a Senate feels confident to take on the House and both government & Senate adopt a strategy of waiting for the other side to crack.
Anything else?: Everything above here pretty much sums it up. But see the end of the piece for more about 1975 for those who don't know the full gory story.
Anything worth copying?: Hmm... Staggered elections, multi-member constituencies and stronger powers with a clearly defined resolution mechanism are all very attractive. Less attractive is the ability to block supply and drive a government from office, but that could easily be denied in the creation of UK chamber. However the model of the chamber is very clearly one of representing clearly defined interests, not revising, and as the UK is not fully federal then these interests would be very hard to define. I can't envisage a chamber based on equal representation from England and Northern Ireland (to take the two extreme) would be workable. Equally I can't see giving the poorly defined English regions an equal number of members would be too popular with Scotland, Wales & Northern Ireland, and even within England there's also the issue of regions with smaller populations like the North East having the same number of members as the South East.
But the principle of an "equal say" second chamber could at least be a way to tackle other issues like the West Lothian Question.
Post Script: I've made many mentions of the events of 1975 so for those who aren't familiar with them, I'd recommend a look at Wikipedia: 1975 Australian constitutional crisis.
And also here's a special bonus feature from YouTube, featuring the satirical character Norman Gunston on that momentous day: