The two houses of the Australian federal government (and state governments) are a combination of the UK and US models. We are basically a Westminster system, so we have the lower house, the House of Representatives, but the House of Lords has been replaced with the the upper house, the Senate, as per the US. The House of Reps has seats allocated by population - each seat represents about 80,000 people. The Senate is allocated independent of population, and was done so to make sure the less populous states were not being constantly overruled by the more populous ones. Both are elected directly buy the people of Australia.
If I remember my separation of powers correctly, the House of Reps is executive, and the Senate is legislative. Any prospective law can be proposed by either house (thought it is usually the lower house which starts them off) but must pass successfully through both to become law. Whichever party holds a majority of seats in the lower house is the government, and the second-place holder is the opposition. The leader of the government is the Prime Minister.
What usually happens is that the government suggests legislation in parliament, gets it through the lower house because they hold a majority, and then has to compromise endlessly to get it through the upper house, because it is very rarely controlled by the government. A party that controls both the senate and the house of reps can do pretty much whatever it likes.
That's Australian Government 101. Hope you enjoyed it.
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