Qualification wise, B Psych is exactly the same as BSc(Hon) or B Adv science.
All B Psych is a B Science that automatically locks you into a Psych major, requires you to maintain a 65+ WAM to "stay in or get demoted to BSc", and gets you a guaranteed honours year as long you've maintained the marks. Keep in mind with B Science all you need to do is maintain a credit average (i.e. 65 WAM) to get into honours.
EDIT: I guess there is that one course in 1st year that only B Psych students can get into, no idea how much of a difference it makes lol
No, I've read otherwise. B Psych students are not guaranteed places in fourth year. They'd still need to compete with B Science and B Advanced Science students at the end of third year.
For 2007 and 2008 anyway, yes, B Psych and B Advanced Science students did need a 65+ average to stay in their program. However, they needed a 70+ to progress to honours, as do B Science students.
B Science entry in the 2009 UNSW Handbook said:
For entry to Honours a student must complete the following Level I, II and III Psychology courses with an average of at least 70%.
In 2009 though, B Psych, B Advanced Science and B Science students all need a 70+ average.
To OP, I suppose as long as you do all the subjects stated
here with an average of 70+, you'll be fine to do Psych. Though, I am unsure if the 70 average applies to level one courses too.
Also, I'm not sure if everything I've posted is 100% correct. But it sounds right, if that helps.