Going back to the definition of data under the Act, what is a "relevant filing system"?

Everyone knows that a filing system is ordered / sorted by number, names, grouping of files, indexes, by Area, by Service, and so on.

If you have collected or are about to collect personal information or are in the process of doing that, and intend to hold or do hold personal data in some ordered fashion, then the Act comes into play. Information does not have to be in a filing cabinet to be caught by the Act. The Act covers any set of information relating to living persons where it is structured in such a way that information relating to an individual is accessible.

The only time the information is not covered by the Act is a collection of papers in a folder having no order or structure. But if that file forms part of a filing system, e.g. It is the "miscellaneous" file, or "sundry" file and contains personal data, then the Act applies.