Is the expression "North Star" (meaning a constant or guide) commonly used in the Southern Hemisphere?
Not a native speaker, but born and raised in South America.
"Guiding star(s)" is an expression that is used often (e.g. "Jane has been my guiding star since I moved here", "I'll follow my guiding stars") and of course, expressions referring to North, since that's where compasses point to, even in the southern hemisphere.
In Portuguese there is even "desnortear" (verb) / "desnorteado" (adjective) which literally means "un-Northed" and translates as "bewilder" / "disorient"