A brief terminology tip: in the EU, as in many other countries, one cannot get a visa while in the country. The document you get while you're in the country is generally called a residence permit (in Germany, Aufenthaltstitel).
The normal procedure is for the foreigner to get a visa allowing entry into Germany. After entering Germany, the foreigner applies for a residence permit.
Nationals of many countries, including Australia, can enter the Schengen area without a visa for short visits of no more than 90 days in any 180-day period. Most of these countries' nationals can use this visa exemption only for such visits; to apply for a residence permit, they need a visa. However, citizens of a handful of countries, mostly "industrialized" countries with large economies, including Australia, are permitted to arrive using the general short-term visa exemption and then apply for a residence permit.
These rules varying by nationality should explain the inconsistent information you've found. Some specific points:
I am wondering how I can obtain entry into Germany...
Show up at passport control with your Australian passport.
...and obtain a student [residence permit] there.
I believe you will do that at the local Ausländerbehörde, but your school's office for foreign students will surely know the specific details. Do be certain to make your application before your 90th day in the Schengen area. For example, if you spend 46 days in Spain and Portugal, you will need to file your application by your 44th day in Germany. To avoid misunderstandings and other crises, it's of course best to file well in advance of the deadline.
I have been told that I can't change a Schengen visa to a student visa when in Germany...
As noted above, that advice doesn't particularly apply to Australian citizens.
...but am confused on how to enter without a Schengen visa.
Also as noted above, this is normal for Australians, who are exempt from the Schengen visa requirement.
Finally, the rules around residence permits and long-stay visas are set by national law rather than EU law, so they will vary from country to country. The fact that an Australian can go to Germany for study without a visa and then apply for a residence permit does not imply that the same process pertains in, for example, France. In fact, I think it does not.