I Attended a workshop run by Umbrella where they discussed this. The facilitator (Dougal Sutherland) had previously run the same workshop with the individuals who had helped to write the HSWA.
He mentioned that when they discussed this topic the writers confirmed that they never intended the Act itself to provide enough guidance on how to manage psychosocial risks. Instead, they expected psychosocial risks to eventually be regulated in NZ. So we agreed that until we have a government who is willing to take this topic seriously and supports the creation of NZ specific regulations, it is likely that outcomes of court cases (i.e. Siouxsie Wiles Vs University of Auckland) will provide the guidance we are currently seeking.