I would say it's never too early to start applying for jobs. To be brutally honest, no matter how well-qualified you are, you're going to be extremely lucky to be offered the first position you apply for; and anyway, even if you are, a potential employer is going prefer that you finish your masters if they don't absolutely require it.
Besides, it's good practise to go through the whole process a few times. Online forms, competency tests, telephone interviews, face-to-face interviews and assessment centres are all common features of a job application and all can be challenging. Getting through the first few stages can boost your confidence and there's absolutely no harm in gaining some experience of interaction with recruiters/employers. This is even worth doing if you're not particularly interested in a position or company, so that when that perfect role does come along you know generally what is expected of you. Especially if you've never worked a graduate role before, you'll likely make mistakes in your applications - learn from them.
Like VBD has said, hiring is a slow process, and applying now serves the double purpose of saving you a long wait with nothing to do post-masters and demonstrating to employers you're enthusiastic, forward-thinking, ambitious and that you can manage time efficiently (make sure you do this: don't compromise on your masters to prepare for an interview!).
Good luck!
