A Bachelor of Arts requires 24 subjects (so, as an example, 4 subjects a semester for 3 years). Doing enough subjects to complete the degree is not the same as doing enough subjects to complete a major, because your degree will have subjects that aren't in your major sequence (ergo 300 points > 150 points).
If you have 75 points to add, it means you can still choose 6 subjects of your liking to fit into the major sequence. The other 75 would therefore be subjects that are either required for the major, or those you've already added.
http://ba.unimelb.edu.au/study/majors/psychology-major/major-structure#major-structureThe above link confirms what I mentioned, showing the subjects under level 2 - compulsory and level 3 - compulsory as all combining to be 75 credit points.
As for the rest of your subjects that aren't the level 2/3 compulsory ones (again, in the link), you need to complete...
Two Level 1 Compulsory Subjects (MBB1/MBB2) = 25 credit points
Two Level 3 Elective Subjects = + 25 credits points
Electives from the Level 2 and Level 3 Major Subjects only
If you're still quite confused, keep in mind that you only really need to sort out your first year of studies to be eligible which just means doing MBB1/MBB2. The rest doesn't really matter for now and you'll have a lot of time to get your head around it.
Link for more info -
https://handbook.unimelb.edu.au/components/b-arts-major-26/printAlso recommend sitting down with a course planner if you're lost.