When I first travelled to Australia, the spraying was done after landing, by Quarantine blokes marching up the aisles in their stubbies (
https://en.m.wikipedia.org/wiki/Stubbies_(brand) ) and knee-high socks.
Nowadays, it’s done by the CC before landing.
Once when waiting for the spraying to be completed, my Aussie seat-mate said “Of course, they only do this when there are Poms on board.”