except that if the screws attaching the switch are fastened to a metal back box and there is a fault then the screws where they appear through the front of the switch can become live hence why we earth the metal back box. agreed that the plastic faceplate of the switch doesnt need earthing hence it doesnt have an earth terminal
See
https://commons.wikimedia.org/wiki/File:Australian_four_gang_wall_plate.jpg and
https://commons.wikimedia.org/wiki/File:Australian_four_gang_wall_plate_-_cover_removed.jpg
for photos of the typical switches/switch-plates in question (With and Without the Cover Plate.)
With the Cover Plate the screws are not accessible.
Without the Cover Plate, the screws are still not accessible (except with a tool), because the screws are set in a small diameter hole, too deeply to be touched by an adult human finger. (6 mm deep)
Further, "back boxes" are used
only in missionary walls (since there they
are necessary) - and, these days, they are usually plastic. (
https://hpm.com.au/product/standard-wall-box/ )
The older metal boxes were not earthed, because the construction of the switch plates and socket-outlets did not require earthing.
(However, some present day metal wall boxes
do have an earthing terminal (
https://hpm.com.au/product/wall-boxes-fire-120-120-and-acoustic-rated/ )
In "plaster board" walls, the switch plates are often affixed to Mounting Clips, which "crimp" over the edge of the cut-out in the plaster-board (
https://hpm.com.au/product/mounting-clips-for-6-4mm-plasterboard/ ) These "clips" are not earthed.
(If the wall plate is later removed for any reason, there is a risk of the Mounting Clip falling off and down the inside of the stud wall. Hence, it is good practice to apply rubber adhesive or silicone between the mounting plate and the back of the plaster board to prevent such a "nuisance" from happening in the future.)
Non-earthed "Mounting Brackets" are affixed to "studs" during building construction (
https://hpm.com.au/product/mounting-bracket-horizontal-stud-mount/ )
Socket-outlets with a (non-earthed) metal cover plate are shown in
https://commons.wikimedia.org/wiki/...Metal_Cover_-_(plus_an_additional_switch).jpg