You can apply to study a single subject (also called a unit) at Monash University without enrolling in an entire course. You will get a statement once you finish your unit, which may count as credit towards a course in the future. CSP funding (government funding) is not permitted for single unit enrolments, so all units require payment of full fees.
For a list of subjects available, see the Monash handbook. Check the unit prerequisites to make sure you are eligible to apply.