From AQIS website--
Why are we sometimes sprayed on international aircraft flying to Australia?
According to the World Health Organization (WHO), certain insects can act as transmitters or vectors of pathogens or parasites that are responsible for spreading a number of human diseases such as Dengue Fever, Yellow Fever, Ross River Fever and Malaria.
The inside of international aircraft may be sprayed when nearing Australia or on arrival to destroy insect vectors that may be in cabin baggage or cargo—this is called 'disinsection'.
Preparations of chemicals currently used in aircraft disinsection are based on two active ingredients, permethrin and d-Phenothrin, currently recommended by WHO. The difference between permethrin and d-Phenothrin is principally one of residual effect; permethrin is a residual pyrethroid and d-Phenothrin a non-residual pyrethroid.
To assist airlines with differing operational requirements, several options for achieving effective disinsection for cabins and holds are available:
- Residual (conducted in the absence of passengers)
- Pre-embarkation (conducted in the absence of passengers)
- Pre-flight and Top of Descent (conducted in the presence of passengers)
- On-arrival (conducted in the presence of passengers)