Hi Ceri,
The Form 80 is a pretty intense form which details all your previous travel details for the last 10 years, employment from Birth, qualifications etc. It asks for details of countries travelled to and dates, places you studied and addresses...etc etc! I would start doing it because if your CO asks you for it and it isn't done, it might delay you. It took my BF and I agessssss to go back through his past, getting all the information together. Esp living in the UK, we travel to Europe a lot and not for a long time so there is not really any evidence of it. Facebook was our best friend trying to work out all those dates. I had to speak to my BF's parents and get info on schooling. The biggest struggle was the employment as it asks for employment from BIRTH!! WTF!! Having 2 degrees my BF has done a lot of part time jobs to get him through his studies so that's been the biggest headache. Your CO might not ask for it, but I would recommend doing it in case it takes a while to collate info. You can find the form here....
https://www.border.gov.au/Forms/Documents/80.pdf
Good luck!
Hannah