i'm doing science/law at usyd and was worried about the job opportunities as well. but from people i've spoken to, it's unlikely that your non-law degree will end up playing much of a role if you decide to go down the law path. and i don't think the major is all that relevant either, unless you intend on using it a lot - i know someone who is a top tax barrister in Sydney and she majored in Chemistry in a combined science/law degree.
I don't know of any jobs that require psych and law though, I did consider that at one stage too. Possibly forensic psych as mentioned above or it may be helpful if you become a criminal lawyer. I'm not sure that a psych major in a science degree enables you to register as a professional psychologist though, i thought you had to do the specialised psych degree.