Some of us don't have a driving licence, quickly becomes habit carrying your passport everywhere

So why don't you get a 'proof of age' card? A driving licence for non-drivers, used by folks of all ages. Much easier to replace than a passport.

http://www.transport.qld.gov.au/Home.../Card_18_plus/

Even though these "proof your age" cards are legal documents that are meant to be worth 70 points or something, I would advise against it - they are a waist of money, each state has their own version and does not recognise the other states, and more importantly if you are a young looking 18 year old most bar staff and security have no idea what they are - I used to work as both, I have only ever seen one in 3 years of bar work and checking I.D's - I would highly recommend that you get your "L" or "P" - provisional driving license or learners permit - they are recognised every where, even in the countries that they were not issued for, they are worth it in the long run even if you have no intention of learning how to drive.

I have quite a bit of experience when it comes to proving who you are