Just engage in as many social activities as possible. Meet up groups in Sydney are fantastic for this purpose. I met new people while I was living in Melbourne prior to moving to Sydney when I had to fly up and work on a clients site every week for a number of months. Easy to make friends with the people you work with first, and then go from there. A lot of people become friends with their share/flatmates in Sydney too.
If you have started at university, the best thing to do is to look into some of the clubs and societies on orientation day. By doing so you will be able to meet new people on a regular basis and also hopefully attend some events.
I agree with Justine and Jess - engaging in social activities, and being part of any institution is a sure way to meet new people. Personally, I participate in my local parish's youth events and am always meeting new people that way!
Most people feel that Sydney is an unfriendly city but in fact Sydney people are more likely to know their neighbours than any other city in Australia and tourists are more likely to get help on the streets of Sydney.
Having lived in many cities in different parts of Australia and in other countries I know that it can be hard to make friends. However there are a few things you can do specifically for Sydney.
1. Get to know your neighbours. Sydney is really a friendly place. My former flatmate got to know everyone in our apartment building by working on his little racecar in the garage downstairs. So sit outside as often as you can, whether you are in the suburbs, in an apartment building or in a terraced house.
2. Get involved in things. Be a volunteer is good, but Sydney siders can be very political so it join a cause.
3. Learn. Sydney people are pushed to achieve and learn so if you study or learn you will definitely make new friends.