"We will not be accepting anybody from the military regime in Burma as a representative of Burma in Australia. That is completely unacceptable."
Downer did not identify the nominee for ambassador, but The Australian newspaper named him as Brigadier-General Thura U Thet Oo Maung, who had served in Myanmar's Shan and Karen states, where rebels have been fighting the government.
Downer said he had made the decision to reject the nominee "a couple of months ago", but that the crackdown on anti-government protesters in recent weeks confirmed the military rulers' brutality.
He said the Australian government believed at least 30 people had been killed in the crackdown - not the 10 that Myanmar state media says - and about 1,400 arrested.