Sadly you've left your run a little late. I
changed $3500 a month ago at $1 >> 106-en,
which is the best I've ever seen. The banks
are OK, I think NAB is a little cheaper than
St George for larger amounts.
If you live in Sydney, there is a money
changer in Westfield Parramatta which always
seems to be a little better than the banks.
The Japanese Postal Savings Bank has a very good exchange rate, but you'll have the evil withdrawing fee that most foreign banks slap onto you. Also, some banks may convert to yen before the money goes to the postal savings, and then you wont make use of their exchange rate anyway.