For non-US residents (like Australians), US withholding tax rate is typically reduced from 30% to 15% under the Australia/US Double Tax Agreement. This rate is dependent on your tax residence and if you have provided a W8-BEN form. As we handle this digitally, a W8-BEN is provided to the IRS on your behalf.
This withholding amount is only on dividends paid and not on share sales. If your shares do not pay dividends, you will not be taxed on them. You are not taxed on profits on your share sales in the US. You will need to declare these in your Oz tax.
Due to the complexities involved with tax, please obtain independent professional taxation advice which take into account your facts and circumstances.