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:
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.