In Australian pre-grant patent opposition proceedings, grounds available to the opponent include lack of inventive step. Lack of inventive step is established if it is shown that the invention claimed would have been obvious to a non-inventive skilled worker, equipped with the common general knowledge (CGK) in the field of the invention in Australia at the priority date of the application. CGK is that which is part of the ordinary equipment of all persons engaged in the relevant art. The nature of the CGK must be established by evidence. An appropriate deponent for this purpose is clearly a non-inventive skilled worker in the field of the invention.