3 things you need to show

1 ) your itinerary - what are you going to do there ?
- if you plan to visit family n friends - then you need an invitation letter (with their address and immigration details - eg, PR number etc)
- on your own visit - then best to draw out a rough itinerary (make sure logical lar, dont say you plan to visit 3 days, but go sampai sana sini)

2 ) how are you going to support your stay ?
- Bank savings - 3 months - show sufficient funds to support your stay there 
- FD (if can) - in case of emergency - how are you going to pay for it ?
- If you are a student - then parents bank account + birth cert showing parents < forgot to add this

3 ) your intention to return
- if working - salary slip 3 months (plus bank account showing credited salary)
- a letter from employer confirming your job and your intention to return to work
if student - a letter from uni/school confirming you are a student there

if taukeh - no salary - then SSm certifications showing that you own the company - and if possible company bank account

*WARNING - always ask your friends what is their visa status and permission if you want to list them ...

DO NOT COMMIT to airtickets before your visa - its already stated in ETA 601 that you should never commit .. cause air tickets, hotels etc restriction on cancellation (in form 1554)
its very fluid, different people have have to show different documents proving their intention ... usually when i do - i will "interview" those people first and get to know them better before accepting ...

I would be travelling to AUS in July/August.
I will be travelling there together with my relatives.
I am trying to apply my visa via the mobile ETA application and now following the link received from the email (like other memeber had mentioned here) on their immigration website.
As I see there is this "Group Processing" option, can anyone advise if joining in as a group and have my application processed together could possibly increase the approval chance or it won't matter, and I should just proceed with the application individually？