Sending a dog from Serbia to Australia request dedicated work.
Actually there is a period and very limited time where all health treatments and tests must be done.
Each test have period of validity. 
If for any reason the dog cant fly, the flight must be canceled and some of tests and treatments must be repeated . 
Other side of work, Import agents in Australia has job to ensure quarantine .
Quarantine will be canceled and the dog need other one if the dog cant fly  for any reason.
Then again the procedure is same according to new quarantine date.