you just need to contact whoever you need to contact - unit convener or the head of that particular department - to discuss your personal situation, relevant knowledge, etc. if there are no pre-requisites, you might be alright.
i'm doing second year subjects in my first year - japanese. i spoke with the unit convener about wanting to enrol in that unit because the first year level was too basic for me and she signed my waver and now i'm all happy.