Depending on your skills or aims you might be able to get a visa and enter the country without a job contract (e.g. if you get a skilled migration visa or a study visa). There are other visa types like the sponsorship visa where you apply for a visa because you have a contract with an employer. 

The Governments of the different States (e.g. Working in SA) provide helpful information for people wanting to migrate from overseas. Check out their Websites to find out whether you might be able to fulfil the requirements for skilled migration or how you could find sponsorship by a local business.

Be aware of the fact that finding a job in Australia might not be that easy. Try to get in contact with people having a similar job profile or living in your preferred region.