This article outlines why. And this article, compiled by a communications lawyer, compares Australia's proposed plan with 29 other countries. Of those, 8 have already ruled data retention to be unconstitutional, and 10 more are currently reviewing their scheme.
In 2013, the United Nations General Assembly adopted a resolution entitled Right to privacy in the digital age, stating that the right to privacy is a human right.
In July 2014, the United Nations human rights chief advised that "Mass surveillance by intelligence agencies is almost certainly illegal under international law, even where it involves collecting but not looking at people’s data".
And yet, the Australian Government presses ahead. To be clear, what they're proposing is that they can force ISPs to collect and keep data ABOUT YOU, for 2 years. Sadly, at this point they can't tell you exactly WHAT data because, well, they don't actually understand what metadata is and thus far haven't been able to define exactly what it is and what they will or won't collect (try not to laugh).
The second part they're proposing is that they can access that data any time they want, WITHOUT any kind of court mandate, warrant or judicial oversight. The argument is that they need these powers to fight terrorism. Or pedophiles. Or... something else scary. But if they want to tap a phone or break into a premises to fight terrorism/pedophiles/other-scariness, they have to get a warrant. A judge decides if the government REALLY needs to tap that phone or break down that door, before the government is allowed to do it.
The only country out of the 29 surveyed where a scheme similar to what the Government is proposing is in place, where they can access data without a warrant, is Poland. And to be absolutely clear - there is NO evidence, anywhere, that shows that collecting data in this manner, has resulted in any benefit whatsoever.
If you read George Orwell's "1984" in school... what's that saying? "Absolute power corrupts absolutely"? No government should have unrestricted access to this much information about its citizens, without some kind of judicial oversight to "keep the bastards honest".
I encourage everybody to stand up for your BASIC HUMAN RIGHTS.