I had a masters interview a few weeks ago, unfortunately I didn't get an offer but I can still tell you what to expect, to a certain degree. I wore skinny black trousers and a nice blouse-type top..if anything go too smart but it doesn't really demand a suit. Just black trousers/knee length skirt and a smart top is fine.
You will probably be asked why you want to do the course and what your future aspirations are. Also, how your undergraduate degree has provided you with suitable background knowledge for the course. They will want yoe to sound motivated and confident in your ability. Also you might be asked about any specific work experience you may have had and what you would bring to the university.
Generally they're quite informal, they don't want to catch you out or anything like that. So good luck, and try not to be nervous, I think thats where I failed to be honest.