The main problem in New Zealand is the common housefly. We treat by lightly misting the inside of your property. This acts as a residual spray and once flies land and come into contact with the product they will die once they ingest the chemical.
The lesser house fly (smaller than the common housefly). They are best known as flying in jagged patterns in the middle of the room and are therefore much harder to knock down as they do not land. The best way to combat these is to install an automatic insect control dispenser.