If you don't do Honours, a Postgraduate Diploma in Psychology can be used as a prerequisite for entry into Masters at most universities.
Also, to answer this question:
Would it be preferable to complete a masters if I wanted to maximise my career prospects?
Are you intending on becoming a licensed psychologist, or are you just talking about improving your career prospects in general?
If you're intending on becoming a qualified psychologist, you
need to do a Masters and some other professional development activities. Not including your undergraduate studies, this can last anywhere between 2-5 years (depending on what type of psychologist you want to be). So yes, a Masters is absolutely integral if you want to break into that field.