Sorry it took me a while to reply.
With regard to whether Broad, General and Cultivated are accents or varieties, I maintain that they are varieties.
Wikipedia and Australian Voices both seem to focus on the accent and ignore everything else, but I really don't believe this is correct. You cannot define a language variety strictly by phonology and pronunciation.
You can have a read of them here if you like:
http://en.wikipedia.org/wiki/Australian_English#Variation_and_changeshttp://clas.mq.edu.au/australian-voices/australian-accentI'll try and give you some examples
Lexicology
-Broad tends to use typically Australian vernacular, and is less affected by globalisation and Americanisms. Slang such as rhyming slang and reductions such as 'bicky' (biscuit) feature prominently. Tmesis is another feature, where words (usually dysphemisms) are infixed within other words, for example, 'sen-fucking-sational'
-General has its own variety of colloquial language, many lexemes of which have been borrowed from American culture. 'Cookie' may be used instead of 'biscuit' for example, and this variety has adopted a lot more technologically derived language, as well as the use of language typically reserved for electronic mediums such as SMS, MSN, Facebook and Twitter. Amongst teenagers, 'ceebs' is practically a codified word. There would not be too many broad speakers who would be familiar with this term though.
-Cultivated is perceived as overly formal and sometimes 'snobby'. It tends to reflect Standard English the closest and its speakers generally refrain from using colloquial language, slang and dysphemism. On the other hand, you may be able to say that this variety often uses various euphemisms to maintain the perception of 'elegance' and 'correctness'.
-Syntactically, Broad and General are more similar. They both tend to feature non standard sentence constructions such as 'me house' rather than 'my house' or 'would of' instead of 'would have'. Ellipsis is also common, with incomplete utterances such as 'need a hand?' [do you need a hand?] frequently used. Another example is the non-standard 'youse' used to differentiate between the plural and singular 'you.' This is also evident in many ethnocultural varieties. The cultivated variety however avoids these types of non-standard features
-Discourse
Broad speakers may tend to use a lot of pause fillers in their speech, and it may be less fluent. Such pause fillers may include 'umm' and 'fuckin' or reptition such as 'I went to the...the....the....' to buy thinking time when speaking. Parataxis is also more likely to feature in this variety, where many simple sentences are joined by a coordinating conjunction, usually 'and.' General has these features too, but probably to a slightly lesser extent. 'You know' is often tacked on to utterances in Broad and General because the speaker is seeking affirmation from their listener or audience. Cultivated speakers tend to look down upon the use of various discourse markers and particles because they disrupt the fluency of what they percieve to be 'proper' English
Hopefully that gives you a bit of an idea.

Also, VCAA have reverted to essay topics which require you to 'refer to at least two subsystems of language in your response', so if you are going to talk about the sociolects of Australian English, you are going to need to deviate from discussing phonology only anyway.
Exploring all the subsystems will allow you to show depth of knowledge, so it is something I would recommend.
If your teacher has said otherwise, then do what they want, but then on the exam change to what will maximise your marks.