Australia has a significantly lower unemployment rate than most developed countries and many businesses are happy to employ travellers from overseas. A work visa is needed for paid employment and you can apply for it through government channels: http://www.australia.gov.au/information-and-services/immigration-and-visas/work-visas. Or you may pay an agency to make the application for you. There is no shortage of them advertising their services on the net.
Fruit Picking: Whether it's apples in Tasmania or bananas in Queensland, backpackers and other travellers play a vital role at harvest time. Information on jobs is available at the fruit growers web sites: e.g. http://www.bananabarracks.com/work.html
Restaurants, hotels and bars are places where many Backpackers find work. Big cities have recruitment agencies specialising in this sort of employment. You can use them but it is not essential. The manager of one agency recently told me that a good way for a traveller to find work is to go door knocking. A few simple rules apply. Don't turn up at a busy time. Ask to speak to the manager. Don't dress in holiday clothes. White shirt/black bottoms and closed shoes are generally acceptable. Hair neat and tidy, including facial hair (guys). Nail varnish either on or off and not broken (girls). Remove facial piercings. Be prepared to offer a free shift to prove yourself. First impressions are crucial. Decisions are usually made in the first thirty seconds.
Other casual employment: Many businesses use casual labour. Large firms, such as cleaning contractors, employ lots of people. Smaller outfits take one or two. Opportunities vary from time to time and place to place. I now live on the Gold Coast and see backpackers carrying advertising boards. Others are knocking on my door trying to sell me thermal lagging or solar hot water systems. Talk to other travellers to get ideas. Big employers can be approached directly or through an agency. That doesn't mean you can't front up in person. Always remember the golden rule: look and act the part. If you want to work as a builders labourer, wear heavy boots and the rest of the gear when you arrive on site. Ask to speak to the boss and be prepared to join the builders labourers' union if that is required of you.
Skilled employment: