Take a look at the free shuttle bus no. 555 - it does a circular route from Circular Quay to Central Station, so you can travel in either direction. It's very handy for Hyde Park, George Steet (the main shopping street), and the Queen Victoria Building (A MUST SEE).

555 runs:

Weekdays: 9.30am to 3.30pm, with a late finish of 9pm on Thursday evenings.

Weekends: 9.30am to 6.00pm

Here's the map: www.131500.com.au/tickets/upload/docs/555map.pdf

If you only once or twice take a ferry or maybe some other bus or train, then you might not even need the MyMulti 3 - those rare trips could add up to less money.

Thanks GovernorPhil for this pointer - I had not stumbled across this one since I always stay outside of the CBD, but it sure will come in handy, not that I know about it.

We lived in Sydney, overlooking Hyde Park, for about 18 months . I used the 555 at least 4 days a week, even for simple stuff like returning my library books to Customs House!
At certain times of the day the bus can get very crowded, particularly between Town Hall and Chinatown, but it's all part of the (for me) great Sydney Experience.