Money Changing, Sydney
The most convenient way to get local currency is to use your ATM card at the many banks dotting along Sydney. You will find the you get better rates than exchanging your currency at those exchange booths.
There are plenty of banks with ATM machines throughout Sydney, including the airport where you may just want to withdraw some money before venturing to the city center.
DO NOT USE THIS REAL GAP
PLEASE SAVE YOUR MONEY AND NOT TRAVEL WITH REAL GAP BUT WITH A DIFFERENT COMPANY
DESPITE PAYING THEM £599 THEY GAVE ME NO HELP WHATSOEVER AND I WAS PRETTY MUCH LEFT ON MY OWN IN THE MIDDLE OF TOKYO.
Anything that they do for you, you can pretty much do yourself.
If you want safety, research your area thoroughly, and bring a fried or family member with you. They will be a lot more helpful than any tour company will ever be and they won't charge you 100's of pounds.
I'm glad you asked me because I can warn you now not to use that company, as they give very poor quality service as soon as they take your money.
For Thai folks, better well-prepared by exchanging your money at home before going on board (or else carry green back). I got A$ for 29 Baht at Bangkok airport. But, it will be 35 Baht for a A$ if I did in Sydney!
These days, many people exchange money by using their bank cards at ATM machines. If you change currency, St. George Bank offers a better rate than the airport currency exchanges. St. George's had a A$1=US 0.5478 with a flat rate A$5.40 commission when I was there. The airport was offering about $0.5731 with a commission (possibly 2%?). For small amounts, there were places on George St. that did not charge commission. If you don't have any Australian currency, taxis accept many credit cards, but not Visa. If you take the bus, you may be out of luck and need to exchange money. (NB: I have no affiliation or own any shares in that bank).