Just thought to share this.
From the SARS website.
When in Portugal, I can trade my SA shares from Portugal without incurring tax in ZA as the act states that the source will be where you apply your mind, wit and labour for your trading activities. SARS uses the Source based tax system.
Another example is a non SA resident trading FX from SA but his funds are in Ireland and his gains are in Ireland. He exercised his "business" activities from within SA so he is liable for tax here.
Had he used a FX manager in Ireland, the gains would not be taxed here but in Ireland as his "business" activities were carried on in Ireland.