As with your tax affairs generally, you need written evidence to support a claim for a foreign income tax offset, and you must keep such evidence so you can provide it to the ATO on request. If we require information that is held overseas we will advise you in writing, giving you time to provide it.
If you use a tax agent to prepare and lodge your return, you need to advise them if you have earned any assessable income on which foreign income tax has been paid and provide them with evidence of payment of the tax.