NIIT (Net Investment Income Tax) is a 3.8% tax on investment income that was passed as part of Obamacare. This tax only applies to investment income if total income is above a threshold that varies based on filing status. For more information, see the IRS website. Following the IRS instructions, US taxpayers are unable to use Foreign Tax Credits (for taxes paid in Australia) to offset NIIT. So Australian taxpayers who are subject to NIIT are paying a Medicare Levy in Australia AND NIIT to fund US health care under Obamacare. This is a clear case of double taxation!
Of interest is this recent post by John Richardson exploring whether the current Australia/US Tax Treaty requires the US to allow a credit against NIIT for taxes paid in Australia.