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I have a bunch of lists for each state in Australia, literally only to separate them.
Is there a way I could - if I combine them in to one list and create segments for each state - that I can then sign people up to a list and assign them to a segment at that point?
You can create a custom domain to store 'States'. Then create segments that matches the state you want.
When you signup users to your list, if you use the 'subscribe' API, you can pass the state name to the 'State' custom field. The segment will then include the user if the 'State' matches the condition in the segment.