Getting a long-stay visa from Germany will not in itself allow you to travel in the other Schengen countries more than you would otherwise be allowed to. (Since you're Australian, you don't need a visa for short stays; if your nationality had been one that needs visas, the long-stay visa would effectively also function as a multiple-entry short-stay visa for the other Schengen countries during its validity period).
However, a long-stay visa does have the effect that the time you spend in the country that issued it does not count for the 90-of-180 day short-stay Schengen clock.
So if you get a working holiday visa you will actually be able to backpack around the Schengen area for 6 months, but you need to space them out such that you comply with the "at most 90 days in any 180 day period" rule. For example, you could start by backpacking for 3 months, then stay in Germany for 4 months, and then backpack for 3 months more.
Note that since there are no internal border checks, your duty to stay in Germany for the middle 4 months is mostly by the honor system. But in order to minimize your risk of trouble, you would be wise to hold on to whatever documentation of your presence in Germany for that period you get, such as accommodation receipts or payslips.
Also remember that if you want to visit some of the EU countries outside Schengen (such as the UK or Ireland) that won't count for the 90/180 rule either.