In some situations there may not be a relevant AGLC rule you can apply to a source. In this circumstance, follow the below advice in 'Sources Not Included in the AGLC' on page xi:
If you wish to cite a source for which provision is not made in the AGLC, you should first reflect on the cardinal principles: clarity and consistency. It may be helpful to analogise with existing rules. However, there is often no single correct way to cite a source and it will be a matter of judgement as to which rule to adhere to. As long as you are clear and consistent, you cannot go wrong.¹
¹ Melbourne University Law Review Association and Melbourne Journal of International Law, Australian Guide to Legal Citation (4th ed, 2018) xi.