And just where did you first learn about this caveat? I've had a US passport for 10 yrs and have never heard of it. Is it an Australian gov't thing only? Thx! :-)

It's written in the information for travelling by plane, also depending on what country but it's the rule for international flights usually. You're only allowed to stay 6 months per calendar year out of your country as any longer, you lose your residency. Unless of course, you start filing for Visa's for extended travel times, and that's done months in advance. Having less than that left on your expiration flags you for possible abandonment AND as others said, makes it so you don't have an expired passport if you do plan to come back in 6 months for any extenuating circumstances. Protects you and them! This fact is 100% tested and true for USA and Canada from personal experience as an immigrant from one country to the other!

Yea, but this is the Australian federal gov't. I doubt that they'll bend the rules for anybody, but high-ranking gov't officials.

Yea that's sadly a thing. I believe it's to prevent people from getting stranded with an expired passport that was still fine when they left the country.

I once heard the 6 month rule explained as a sort of buffer, in case something goes really wrong and you are delayed leaving (medical issues, legal trouble, whatever).