The decision in Hamad v Q Catering Limited  NSWWCCPD 6 (15 March 2017) emphasised the obligation to obtain expert medical evidence connecting the psychological injury to the actions relied upon by the employer when a matter encompasses possible multiple causes for the injury.
In Hamad it was stated by Deputy President Michael Snell at  that:
“… There may be cases in which causation of a psychological injury can be established without specific medical evidence, for example where there is a single instance of major psychological trauma, with no other competing factors. The need for medical evidence, dealing with the causation issue in s.11A(1) of the 1987 Act, will depend on the facts and circumstances of the individual case. In the current case, as in most, there are a number of potentially causative factors raised in the applicant’s statement and the medical histories. Proof of whether those factors, which potentially provide a defence under s.11A(1), were the whole or predominate cause of the psychological injury, required medical evidence on that topic. The extent of any causal contribution, from matters not constituting actions or proposed actions by the respondent with respect to discipline, could not be resolved on the basis of the Arbitrator’s common knowledge and experience”.
In short, Snell DP held that because a psychological injury can be caused by the cumulative effect of events leading up to an ultimate and dramatic decompensation and the relative causative role played by those events is not a matter that can be determined by reference to ordinary commonsense and experience, medical evidence is required which addresses those relative causative contributions before a finding as to what “wholly or predominantly” caused the injury can be made in favour of the party carrying the onus in relation to a defence under section 11A(1).
In Hamad the respondent employer was unable, on the available evidence and in the absence of any medical evidence dealing appropriately with the topic, to discharge its onus in proving the worker’s psychological injury resulted ‘wholly or predominantly’ from its reasonable action to be taken or proposed to be taken with respect to discipline. As such it is now clear that factual evidence alone will not always be sufficient to make out a s.11A defence. Where factual evidence is adequate, it will usually be cases in which there is an allegation of a single event that has given rise to psychological injury. These cases are rare.