Despite the best efforts of commentators to complicate the answer to this question, it is disarmingly straightforward.
The majority followed Board of Bendigo Regional Institute of Technical and Further Education v Barclay (2012) 248 CLR 500 in recognizing the critical distinction between the characteristic protected by s.340 or s.346(a) of the Fair Work Act 2009 or the act that constitutes participation in an industrial activity for the purposes of s.346(b) on the one hand, and on the other hand the thinking of the decision maker with respect to that act.
The protection given by s.346 is not against adverse action taken because an employee engaged in an act or omission that was a protected industrial activity, but instead it is protection against adverse action being taken by reason of that act or omission having the character of a protected industrial activity. That is because ss.340 and 346 are not concerned with causal connections, but with the reasons for the decision maker’s actions. So, an employer can lawfully dismiss an employee because the abusive content of a sign that he brandished on a picket contravenes the employer’s code of conduct, but if the dismissal had instead been substantially and operatively motivated by the fact that the picket was an industrial activity organised by a union, or by the fact that the views or interests represented or advanced by the abuse were those of a union, then the dismissal would be unlawful.
The same distinctions were recently applied to s.340 by a strong majority in CFMEU v Endeavour Coal Pty Ltd  FCAFC 76 at - (Jessup J) and ,  and  (Perram J). An application to the High Court for special leave to appeal is pending in Endeavour Coal.