There is an exemption under the Motor Vehicle Regulations from Registration for Ag Machinery and/or Implements when they are being towed or on display at 'prescribed events'.
prescribed event means—
(a) the annual Royal Adelaide Show; or
(b) a country field day event; or
(c) a boat or motor show; or
(d) any other exhibition, fair or show;
But...you must have appropriate Insurance in place, see below for further details: