If you work with a temp contract, you can rest assured that you earn the same wage as your coworker with a permanent contract. That's what we call the principle of equal treatment. The principle of equal treatment means that you are entitled to the same salary as if you were employed under a permanent contract.
Wages do depend on many factors, such as your seniority (ie, your experience). If you compare your salary to that of your coworker, it's possible that there is some difference due to those factors. Fortunately, your wage cannot be randomly chosen by your employer. There are a few agreements that have been laid down by the law. You can check those agreements, or scales, on www.minimumlonen.be.