From JeppView, State Rules & Procedures - Australia, p 939.
7.1.10 Pilots must select the transponder to STANDBY before effecting an SSR code change and returning the transponder to ON/ALT.
NOTE: This action is required to prevent possible loss of displayed aircraft position / label infor-mation and possible misidentification of aircraft in automated Australian ATC systems due to tem-porary selection (while effecting the change) of a code already in use.