Technically speaking, it is the issue whether these shares transactions constitute a trading in nature of itself. Long-term holding or short-term trading and its frequency. An issue of substance over form.
For shares dealing, it is a normal practice that if the account holder is a natural person and not of a legal person, limited company, apparently, these transactions could be considered as not for trading and not with a view for profit in nature. Just look around that there are so many transactions carried out by individual investors every day with gains or losses.
IRD will normally not eager to know the source of this trust money for the purposes of tax law. It is the issue of doubt why you have such a big sum of money likely earned from chargeable income. The onus of proof falls on you to prove the source or otherwise.
The issue of money laundering is not of a tax issue but of the concern of the Monetary Authority.