Some more information would be useful, please, @Xing , but it might be far better if you sought advice from a tax professional (or two) familiar with IRS and ATO obligations.
- Are you (still) an Australin resident for tax purposes?
- Are you a US citizen and therefore also obliged to complete IRS returns?
- If both, where do you reside as this may affect which jurisdiction has first rights to taxes?
- Has you SMSF invested in Australian funds holding US assets or in US assets directly?
- If you have dual tax residency, have you considered that you may need to determine CGT separately for each country given the rules are quite different?