Most airlines flying to Australia pre-treat their aircraft using a long-lasting treatment. This eliminates the need to spray (which was common a decade ago). A few airlines spray instead of pre-treating. The health hazard is the same in both cases (believed to be zero). UA seems to spray often, fwiw, while QF never sprays (all their planes are pre-treated). This isn't unique to Australia, and many US domestic planes have chemical treatments in the passenger cabin. Snakes on planes, anyone?