When is "vesain tal umotor" said in the Southern Hemisphere eg Australia?
I'm assuming you are dealing with the weather and seasonal-related issues, and not the possible International Date Line issues (which would make a fine question in itself).
From this article by Rabbi Yirmiyohu Kaganoff shlita:
The Gemara (Taanis 10a) concludes that in Eretz Yisroel one begins reciting ve’sein tal umatar on the Seventh of MarCheshvan, whereas in Bavel one begins reciting it on the sixtieth day after the autumnal equinox.
This practice is followed in all of chutz la’aretz, even in places where rain is not seasonal, or where rain is needed earlier — although the precise reason why all of chutz la’aretz follows the practice of Bavel is uncertain (see Rashi and Rosh to Taanis 10a; Shu”t Rosh 4:10; Tur and Shulchan Aruch Orach Chayim 117).
He then goes on to cite some opinions that say that the date would vary based on national conditions, but says that we do not follow such opinions (see there for lots more - this is an oversimplification).
Indeed, my Artscroll Siddur simply makes the distinction based on Chutz Laaretz vs. Eretz Yisroel.
And for practical concerns: according to my local calendar and here, in 2011, we start saying it by Maariv on December 5.