In order to legally work in Australia, citizens from most countries require a certain visa. You should contact the Australian Embassy in your home country to get more information on the Visa which is appropriate for your circumstances.
I’m from the UK and the main purpose of my visit was to travel, not work, so I needed a Working Holiday Visa which allowed me in the country for up to 12 months. The visa allowed me to work during that time but I could not work for a single employer for longer than 6 months.
If you intend to spend a few months in Sydney and experience all it has to offer, finding a job is a must. The good news is there is an abundance of jobs in Sydney. Many of these jobs are in coffee shops, bars and restaurants where you can expect a wage of about $20 per hour.
There are lots of employment agencies that can help in your quest for the perfect summer job (many of these also send your CV to other agencies) but you should also get out there and look for yourself. There are always notices in shop windows asking for staff and you are sure to find what you are looking for with a little perseverance. The trouble with using agencies is that due to the number of travellers who use them, the chances of them finding you a job can be low, especially if you have a specific role in mind.
It is also possible to get an office based job while visiting Sydney. Lots of travellers are recent Graduates who want to get a job that will look good on their CV and that expands upon/directly involves their area of study. Employment agencies can help here too but you should look for agencies who recruit for specific sectors e.g. IT, Design, Management etc. It is also worth while contacting employers directly, ideally before you leave for Australia.
There are advantages however of job hunting from within Australia. While here, you will find smaller companies that you may not easily find on the Internet. These companies tend to be more likely to offer short term employment to travellers and you may also gain a greater level of responsibility than you would do at a larger corporation. Being available for face to face interviews is another big advantage that should not be underestimated, most employers hire based on a candidate's personality first and their qualifications, second.
Employment websites are another good way to find work. Two popular websites are Seek and Gumtree.
www.seek.com.au
www.gumtree.com.au