You say I have no idea, yet, you post no links showing what amount of debt the ATO will, or will not chase.
How about you stick to some facts, and not get personal?
The whole system is migrating to computers doing the heavy lifting. To a computer, a debt is a debt, thus, a letter is generated. Once that letter is in play, one is on the ATO merry-go-round.
Do you really think expats can hide from their ATO debt because it's small? Once again, post a link to show what the ATO calls "small debt."
Post a link, or STFU, because it's you has no idea.