The BMW Oz website now has a twitter page...
Anyway, on twitter they have a link to a BMW Group page Where we build our vehicles
which lists the plants where all BMW models are currently built...
When you click on the 3 Series Sedan, it lists 4 plants including Rosslyn, South Africa where a lot of the Oz Sedans seems to originate from. The 3er Touring is built in Munich, with both the 3er Coupe & Convertible in Regensburg.
For info on all other models, check out the link above.