If you are South African resident (which it sounds like you are) you will be taxed on your worldwide earnings. However the current tax threshold is R75,000 for the year which means that if you earn below this amount in a tax year (1 March 2016- 28 Feb 2017) then your earnings won't attract tax anyway.
It won't be taxed when you exchange it to rands. If you are earning above the threshold in a tax year, it is up to you to register for Provisional tax declare it to SARS and then file a tax return.
Please Register for TaxTim and let us assist you with your tax return completion and submission to SARS.
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