I'm not saying you're wrong, but do you mind providing a source for that figure?
Additionally, who are our export markets for aluminium and who are our major competitors?
The economics of the aluminium smelting industry have been in the public domain for 30 years now with a great deal written on the subject by industry, environmentalists and the electricity industry. Bell Bay (Tas) smelter historically has attracted the most attention, although there has been plenty written about the industry in Victoria as well.
In short, the smelters buy bulk wholesale electricity at or near the cost of production (about $40 per MWh, also a widely available figure) and this amoutns to 25% of the cost of smelting. As I said, those figures have been in the public domain for a long time now.
Also widely known is that coal-fired generation (from a typical operating black coal plant) emits about 1 tonne of CO2 per MWh. That figure has also been in the public domain from industry, government and environmentalist sources for many years now.
The rest is simple maths. A $25 per tonne tax works out to about a $25 per MWh increase in generation costs from black coal, increasing the total cost from $40 to $65 which is roughly a 60% increase in the cost of electricity, which represents 25% of smelting costs therefore the carbon tax is a 15% increase in the total cost of smelting.
Note that the other major costs are raw materials (alumina, the "half way" stage of aluminium production and also an energy intensive material to produce), petroleum coke, labour, building the plant in the first place etc. There's only limited scope to reduce those costs.
As for competition, in short it is anywhere that has cheap energy. South Africa, Brazil, Russia, Middle East, Australia, Canada etc are the relevant countries. Here's a list of operating smelters
http://en.wikipedia.org/wiki/List_of_aluminium_smelters See this one also
http://en.wikipedia.org/wiki/List_of_countries_by_aluminium_production
Historically, smelters tended to be built where cheap hydro-electricity was available. No surprise then to find the oldest smelter in Australia (1955) is in Tasmania and that there are lots of them in Canada and one in New Zealand too. In later times coal became a competitive alternative, hence the construction of smelters in Vic, NSW, Qld and plenty of places overseas.
Will the smelters really close? At one point Japan had a large aluminium smelting industry. Then OPEC jacked up the price of oil the Japanese were using to generate the power. There is only one very small smelter remaining in Japan today, the rest being long gone.
Much the same happened in the USA during the early 2000's when that country's energy costs became uncompetitive. If a business becomes cash flow negative on a permanent basis then ultimately it must close.