Question for the smarter ones than me with accounting...

Question for the smarter ones than me with accounting knowledge.

When a ASX IHL LT holder tansfers (due to Redomicile to the US) their full paid ordinary shares to the NASDAQ what tools do the IRS use to determine CGT when a sell order parcel was purchased on the ASX.

Does the IRS require Australian holders to lodge an American Income Tax Return.

If so will there be any concessions when lodging an Aust Tax Return.
Thx in advance.