In WA, you cannot attach any documents to a mortgage document in PEXA. You can add text in the text box within the mortgage document itself, but it is limited to 4,000 characters. This is a Landgate specification.
Options are to register a MCP (Memorandum of Common Provisions) for the T&Cs and refer to that when lodging the document, or add them in to the loan agreement and refer to this in the free text box. However, as this is a legal decision, we can’t advise further on the best option/enforceability.