The Senate or House of Review is the secondary organ of legislative government. It may also initiate bills or Acts and in certain cases does not have to pass them to the Lower house for review (an example of this is the standard Supply bill). The Senators are empowered to form committees of review to review the actions of government (this is always strenuously resisted by governments of course) and to review the results of legislation.
After a bill has been passed up to the Senate and reviewed, the Senate may opt to send the bill back for further review; in such cases the convention is that if the review fails the Senate for a second time, the bill has failed. By further convention, this is a trigger for the government to dissolve parliament itself since it is considered a failure of government to further the will of the people. This is a euphemism for an impasse because usually the government does not control the Senate and cannot pass legislation as it wishes without barrier (or in most cases, debate). For example, until early 2006, the Howard government had little control of the Senate; before then it had several triggers to election (and/or double dissolution) but since it regained control via some by-elections these triggers became moot.
The Senate is thus meant to be a check-and-balance on the executive branch's legislative agenda; in practice, it either provides the Opposition with political ammunition or it is a rubber-stamp for Government. Neither scenario serves the public interest well.