Hi Lily - This can vary depending on the timing of the processing of your application and the nature of the documents to be attached. Generally attaching documents (depending on the document) within a few days of lodging the application is not an issue, however it's important to remember that there are basically 3 types of info/documents that can be part of an application:
Valid application requirements - if a doc or information item is in this category, it must be present with the lodged application or the application will be declared invalid - basically as if it never happened.
Time of Application requirements - a doc or information item that must be present at the "time of application". For instance, IELTS requirements are often time of application requirements - you must have sat the IELTS test at least 1 day prior to applying, but not more than 3 years before applying, etc. You may send the IELTS test score document when you receive it 2-3 weeks after applying, however it must reflect the proper test date, etc.
Time of Decision Requirements - these are requirements that must be met prior to the decision being made by DIAC - often things like medicals and police checks fall into this category, so there's much more room to submit these docs after application (sometimes long after application) but prior to decision.
How to tell whether a requirement is a Valid Application (aka "Schedule 1") requirement, TIme of Application, or Time of Decision? Only one place - the legislation - specifically the Migration Regulation 1998 Schedule 1 (Valid app) and Schedule 2 (Time of App/Dec).
Sorry for the long response, but hope this helps better understand this area of migration law.
