I am not a veterinarian, but Newcastle Disease feels like a strong contender based on your description, your general location, and the mortality rate you are experiencing. You may know Newcastle disease as Ranikhet disease - same thing, different name.
Newcastle Disease (aka Ranikhet disease) affects chickens more severely than turkeys, and ducks can carry it without showing symptoms.
Signs/Symptoms:
Often, the first sign of infection with a virulent strain is sudden death. Other signs can include gaping, coughing, sneezing, weakness, depression, greenish diarrhea, swelling of the face, leg paralysis, and soft-shelled/shell-less eggs. Milder forms of the disease may just cause respiratory issues. Not all symptoms are seen with all strains.
Treatment:
Unfortunately, there is no treatment. You can try to prevent infection by vaccinating your chicks for the disease and taking biosecurity measures to reduce exposure. These measures would include keep birds on your property, preferably in an enclosed area where other birds won't come into contact with them.
How does it spread?
Spread is usually via the air (aerosol), or through birds eating contaminated feces. Transmission through the egg does not occur, however, young chicks can be exposed to virus that is on the egg shell resulting in the development of disease. Humans play a central role in the spread of the disease. The virus is also very resilient and can survive for long periods (up to 12 months) within feces and dust in a poultry house. As a result, as with most diseases, cutting corners during cleaning and disinfection can have disastrous results.
In the United States, we have veterinary diagnostic labs that will run testing on dead birds. I'm not sure what is available in your country, but I would look for veterinary diagnostic labs, veterinarians, or even universities in your country that will work with poultry to identify what you are dealing with. Samples (including whole carcasses) can be mailed if packed/stored properly, so you may be able to find somewhere you can't drive to, but you can ship to.