My undergraduate degree is from outside the UK, also with a different system. Many online forms (for the GDL, training contracts etc.) have a chance to give your classification as “other” and then write something to explain.
I don’t know what a 70% average means in the Australian system. Neither will potential recruiters. So you need to give some context. Can you come up with a rough equivalent to the UK degree structure? I was in the top 3% of my graduating class, so I say my degree is “equivalent to a UK first class honours degree”. I hesitate to mention Wikipedia, but they *do* have some information (drawn from universities like the London School of Economics) about international comparisons of degrees (http://en.wikipedia.org/wiki/British_undergraduate_degree_classification#International_comparisons). It doesn’t explicitly mention Australia, but maybe some of the footnote links would be helpful?
I may be wrong, but I’m guessing that most UK law firms don’t have a lot of experience understanding how Australian degrees equate, so the more you can do to give them context, the better your propsects. I don’t think having an Australian degree would hinder your propsects per se, but you have to be able to sell it.
To get a place on the GDL, you will need to get SRA approval that your degree is equivalent to a UK undergrad degree (a “certificate of academic standing”). There is information about how to do this on the SRA website and when I did it in 2007, there was no charge for the service. I did nhave to send a certified copy of my degree certificate and academic transcript. If the SRA approve you, there should be no problem getting on the GDL.