What course are you interested in? it depends actually. but for
Poly students, with the exception of Uni Mel, the others in
Australia , as long as 75% of your grades are B and above, u shld
be able to gain entry into the more reputable state Unis (i am
generalising this to commerce an engineering courses)
Both questions are related, so i'll answer them together.
Getting a pass isn't difficult. Good grades is definitely another
issue. Ignore the comment on your English proficiency, as some of
other others have pointed out in Aussie, u'll have people from HK,
Jap, etc. If they can cope, there's no excuse for you not too. so
do not worry abt that. 
My advice is this, try to get into state Unis or the group of 8.
If you need more info abt wad courses are good in which Unis, PM
me. i'll gladly tell you more.
Lastly, IDP holds very useful seminars. you can drop down to their
office at Grange Rd (i think) and they can also help.
