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The form W-8BEN is for you to claim treaty benefits with regard to the earnings you receive from your investing. The tax treaty between the U.S. and Australia has a beneficial treatement of this income. Without a determination by the IRS, if you do not have a W-8BEN on file claiming the tax benefits, withholding tax from your investment earnings could be as high as 35%. However when you file this form, the agent who does withholding has to withhold at the treaty rate.
The following publication has a chart that lists the treatry tax rates for various categories of income. Your concern is: interest, dividents, and capital gains.
capital gains, by treaty are 30% and Dividents are at at 5 or 15 percent depending on source; and interest is at 10%.
Short term capital gains are taxed as regular income, except that the treaty rates will apply.