Throughout Australia, mosquitoes can be found in every state and will rapidly breed where there is an open fresh water supply. Rainwater collecting in pot plants and trays, buckets and the like create fertile breeding grounds. A water tank that isn’t properly screened is just asking for trouble.
When purchasing a strainer or screening the aperture size (holes) on the meshing need to be less than 1mm to stop mosquitoes. They should also be made of stainless steel to prevent holes developing through corrosion.
If you notice some mosquitoes already buzzing around inside your tank, then you can add a small amount of domestic kerosene or liquid paraffin. The Department of Human Services in Victoria recommend one teaspoon of domestic kerosene in a 1,000 litre water tank or 3 teaspoons in a 10,000 litre tank (double these amounts if you use liquid paraffin). Do not use industrial or commercial kerosene, and you should verify ensure adding these substances are alright for your tank.