Here's a link to a good micronation clearing house: What Is A Micronation?
Some of them are plainly daft, some are..."eccentric" is the most polite term...some are earnest in their beliefs. Depending on who you're talking to, some of them take it quite seriously, indeed.
As to "legitimacy" in re the Australian constitution, I believe that covers forming a new state within Australia - most countries have similar laws on the books...It certainly doesn't cover questions of direct secession.
In the link above, see the link for the Montevideo Convention...