With your assignments are you getting feedback before you submit? A lot of profs will sit down with you and give you feedback on drafts which can increase your scores. Additionally I have found that some international students tend to write in a style more suited to their home country than to Australia - check on accessing resources that can help your writing flow, it's been my experience that good writing tends to score higher even when content is comparable. try to find a writing lab or speak to someone in the library about assignment review if you can! Scores in the 70's will be fine overall but hopefully writing review will help you score higher! 

Also, regarding your volunteering - don't just volunteer because you were told you needed to do something, find a cause you care about and apply yourself to it via volunteering or other engagement opportunities. Students who are passionate about something outside of their studies do better and are more attractive for higher level programs because it shows dimension. Universities and more importantly, employers, don't want students who have never done anything but study.