Composting toilets are designed to keep flies away, and with proper maintenance flies shouldn't be a problem. If, however, you've found flies buzzing around your toilet, you need to deal with them sooner rather than later.
A well-designed and properly maintained toilet shouldn't have any unpleasant odours. If it does, something is wrong. Here are a few reasons as to why a composting toilet may start to generate odour, and ways to fix it.
If you have a self-contained system that requires frequent emptying, it is WCTNZ recommended practice to set up a secondary treatment system in which the compost can mature prior to burial.