You are mixing up two linguistic categories: Dialects and varieties. Neither British English nor American English nor Australian English are English dialects. They are varieties of English. And of course, beside those varieties there are lots of english dialects.
The same is true for German. There are three standard varieties of German language:
- German German with about 80 million native speakers
- Austrian German with about 8 million native speakers
- Swiss German with about 4 million native speakers
Those are standardized Languages, which means, that there are clearly defined rules for each of them, and Language is taught in schools according to those rules. So asking for "differences among the varieties of Austrian German" simply makes no sense. By definition within one standard there can't be any differences.
Most of those rules, that define this three variations, are equal, specially orthography is equal in all three variations, except that the letter »ß« (named »Eszett« in Germany and »Scharfes S« in Austria«) does not exist in the Swiss standard. It always is replayed by »ss« in Swiss German. Differences in orthography between German and Austria German are very rare (like »Geschoss« in Germany and »Geschoß« in Austria because the o in both words is spoken short in Germany but long in Austria).
But grammar differs in the three variations (tenses even have different names in Austria and Germany because they are used in different ways), and you find noticeable differences in vocabulary and in the usage of same words.
Dialects are not standardized. There are no orthographic rules for dialects, since dialects are spoken, not written (except for poems, songs and similar art). The grammar of dialects is not taught in schools, but the grammar of variations is.
The big difference between English and German is, that in English the regions of the three big standard varieties are geographically separated. They lay on three different continents. But the three regions for German varieties are neighbors. So, while in english each dialect can be assigned to exactly one variety, this is not possible in German. The towns Passau (In Germany) and Schärding (in Austria) are only about 10 km apart. You will not hear any differences in the dialects when you listen to the people living there. But in the schools of Passau students learn German German, while the students in Schärding learn in school a different variety of German: Austrian German.
I have written more detailed (and in German Language) about this topic here: Unterschied zwischen Varietät und Dialekt
Most of the dialects in Austria (all regions except Vorarlberg) belong to the group of Bavarian dialects (in linguistics there is no such thing like "Austrian dialects"). This is a family of dialects spoken in most parts of Austria and most parts of Bavaria (a province in the south-east of Germany). But since there are 12 million people living in Bavaria and only 8 million in Austria, this group of dialects is not called "Austrian Dialects" but "Bavarian Dialects".
In Vorarlberg (the west-most province of Austria) there is no bavarian dialect spoken. Those peoples dialect is very similar to what is spoken in Switzerland, and this dialects are called "Alemannic dialects".
In Wikipedia you find an article on German dialects: Deutsche Dialekte
There is also a article in English Language: German Dialects
More about the group of bavarian dialects. German Text: Bairische Dialekte
In English: Bavarian language
Alemannic Dialects (spoken in Vorarlberg, the west-most province in Austria): Alemannische Dialekte
English Version: Alemannic German
About Dialects in Austria: Dialekte in Österreich
(Sorry no english version)
A text written by me about the confusion between dialects and standard variety when talking about the Language used in Austria: Was ist mit »österreichisch« gemeint?