The situational judgement test is the first test I took as well. There is little to do to prepare, the main thing is to think of the competencies when answering, and to try and pick the most diplomatic answer if the question is related to solving a dispute or other tricky work situation.
Occasionally, they may throw in a question where the ‘most obvious answer’ is wrong. This is most often related to work-life balance situations, for example: ‘A manager asked you to stay behind late to do non-urgent work on a client project. However, you have an important pre-arranged dinner with your family this evening. What do you do?’, along with a possible response along the lines of “cancel the dinner and stay late to complete the work, as completing the project is more important than my social life”. This would be a trick response, as although that option might appear to be the most likely one that they might want to hear, they realise that not many people would actually do that in real life. The ‘most correct’ response is something along the lines of “explain to the manager that you would not be able to complete the work this evening, but try to arrange a time tomorrow to ensure the work is completed”. This is an attempt to screen out the ‘liars’, as well as cut out those who may have significant work-life balance issues.