I'll be bringing cash for a 20 day trip, and I'm changing it all to my port of departure (Australia).
So now, I have lots of yen in my wallet.
Any tips on how I should handle the money while moving around? I'll be staying in hostels (J-Hoppers, Hana Hostel, etc), and I'll mostly be travelling around by public transport and walking.
Is it safe to keep some in my bag back at the hostel? Should I bring it all, or should I bring only what I think I'll use for the day?
Japan is an extremely safe and largely cash-based society. As I remember, if you use a cash machine the smallest amount you can withdrawal is the equivalent of 200 USD. Also, there is virtually no crime. So it is quite normal to walk around with a lot of money in cash.
Second, it is no problem to pay for a small thing with a giant bill. In some countries if you pull out a £50 note or a 100 USD bill to pay for a magazine, there is no end of scrutiny, as though you are doing the cashier a personal slight. In Japan, it's perfectly fine, except at some vending machines. So make sure you get large bills when you change your currency---to reduce the physical bulk in your wallet!
Personally, I would not leave this kind of money unattended in a hostel. That is simply asking for trouble. There will be no disadvantage in keeping it with you, except I suppose the possibility of losing it.
If you do keep it on your person, use the typical precaution of breaking it into bundles and keeping most of it somewhere else on your person or in your bags where it won't be discovered every time you pay for something.
All of this said, even though Japan is largely cash-based, I think it is more sensible to arrange with your bank a card with a 0% forex rate, pay with card when you can and make cash withdrawals as you need them.
For starters don't exchange it in Australia, you will get a better rate exchanging it in Japan. (hard to tell from your post if you have or will do this, since you used different tenses in each sentence)
Bring it all in AU$100 notes, so you have less bills to conceal. Then exchange it in smaller amounts as you need it, not all at once.
Personally I would keep the cash with me, rather than leaving it in my bag in a hostel or other group share accommodation.
But ultimately, you need to weigh the risk of losing your cash to theft vs the fee(s) imposed on ATM withdrawals by your bank.
The best advice, even though it requires more preparation (the first time) than simply taking cash, is to get a Charles Schwab High Yield Checking account. You will get an ATM card that you can use at any 7-11 in Japan, and 7-11s are plentiful in Japan. It's possible to use at other ATMs as well, but I know for a fact they work at 7-11.
The benefit of this method is that there is no fee for the exchange (as there is when doing it with cash at an exchange service), nor is there any ATM fee. It doesn't cost to open the account either.
I use this method whenever I go to Japan and I prefer it over cash.
