Haven't seen that before! Is this a random mutation or a recognised variety?
Or did this one just happen to perch directly under all the other magpies?
This is a quite common variation. Most of the magpies in my area (south of Canberra) are white backs. There are various zones splitting the forms east/west and north/south throughout the country. Quite a bit of research has also happened around this topic. There is even a recent PhD! https://research-repository.griffith.edu.au/bitstream/handle/10072/365743/Dobson_2017_01Thesis.pdf?sequence=1&isAllowed=y
There are 3 main plumages of Magpies. The White-backed subspecies of Magpie is most common in southern Australia, with the regular Black-backed subspecies occurring throughout the rest of Australia and the Western subspecies occurring in Southwest Western Australia. The White-backed Magpie is at its northernmost occurrence in the ACT area so it is generally more common to see Black-backed Magpies locally.
These other CNM sightings are of White-backed Magpies:
https://canberra.naturemapr.org/Sightings/4201776
https://canberra.naturemapr.org/Sightings/3410321
https://canberra.naturemapr.org/Sightings/3392383
https://canberra.naturemapr.org/Sightings/3360763