Trustees are subject to what are called fiduciary duties. This is duty of loyalty.
Fiduciaries must always act in good faith and not for their own benefit. This means that a trustee must not put themselves in a position where personal interests conflict with their duty to the trust's beneficiaries.
A trust is not the only situation where a fiduciary relationship is present. Company directors/shareholders, lawyers/clients, doctor/patients are all in fiduciary relationships.
Generally, fiduciary duties are formally recognised in a written document, for example a trust deed or a Will. They can also arise in a business context, particularly joint ventures.
Fiduciary duties will occur regardless of what the parties intended. What matters is whether “one party was entitled to repose trust and confidence in the other, and did so.” *
* Chirnside v Fay  1 NZLR 433 (SC).
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