Prior to 2024, any remittance of funds not earned in the year of remittance, was non-assessable. In practice few paid
From 2024, remittances of savings accumulated prior to 2024 are non-assessable and remittance of earnings after that time, regardless of when remitted, are assessable.
The above had the predictable effect if decreasing remittances. So now there is serious talk of changing it going forward such that earnings remitted in the same year as earned will be non-assessable.
In all cases, you do not attach any sort of proof. You just complete your tax return showing such income as is assessable under the current rules. Only in the (very unlikely) case that you are audited/questioned would you need to show any sort of proof. In which case just showing that you had at least that amount in your possession (bank account,bonds, etc) before 1 January 2024 would likely suffice to support a claim that the funds were savings rather than income. (I've yet to hear of anyone needing to do that. )