Control of the senate is relevant when it comes to talking about the effectiveness of parliament, which is part of the study design. If the same political party has control in both the senate and the lower house, this could reduce the effectiveness of the senate as it could become a 'rubber stamp', simply confirming decisions of the lower house and not scrutinizing bills as thoroughly as possible. This happens because most of the time, members will vote along party lines. Conversely, if there is a hostile upper house (that is, the government does not have control of the senate) there is the potential for the senate to block all bills passed by the govt, limiting their ability to fulfil their legislative program, which they have been elected to do. If you needed an example, you could use the one where the senate blocked the money bill of the Whitlam govt (sorry i've already forgotten dates and shit)
The term 'balance of power' is also part of the study design (i don't have a copy on me, but if you don't already have one, get it off the vcaa website, ESSENTIAL) Basically, the constitution divides power up between the states and the clth, and this 'balance of power' can be changed by things such as HC decisions or referendums etc, but you won't learn about this until later in the year I don't think.
Hope this helps :/