she's gone. 4th bird we have lost in the last year for no reason

Can you take or send the body to a vet lab or agriculture of ministry for testing/necropsy?
I'm not sure what you call the services in AU, hopefully @Lillith37 might know.
Here in the U.S. each state has lab services, most all countries have some type of lab testing available to those who keep poultry and livestock. Generally the body needs to be kept cool (refrigerated, not frozen) and is either shipped or taken for testing. A pathologist will perform testing/necropsy and give you the results. With this information, you will know more about what's causing the losses and if there's treatment options or if any changes can be made.