basically they'll test your understandings in your study. A power plant company will likely to test your undestanding on basic thermodynamics principle (first law, second law) , steam cycle, gas turbine cycle.
Sometimes they'll will ask for your theoretical knowledge on some common mechanical machineries e.g pumps, fan, boiler, heat exchanger.
some companies also provide a written test prior oral interview. The written test usually consists of the questions to test your knowledge in basic engineering/science concepts (electricity, materials, chemical, strength etc). Usually the questions are not focused on calculations but just asking for explanation of the concepts.
Some interviewer will interested with your final semester project. Do bring a copy of your project report and present to them briefly on the project scope, results, and conclusion of the project.
the tricky part is salary. Unlike a semicon company, a company that heavily involved in mechanical engineering (power plant, O&G etc) will not usually reward a fresh grad with high salary. They value experience and practical knowledge instead. So, your expected salary range is between RM1.8K-2.5K but RM2.5K usually reserved for MNC co (shell, schlumberger etc) and for those candidates with exceptional quality (academic, theoretical understanding of their field).
I knew this becoz I went for such interview for the same post in a power plant company and..HIRED!!!