The former deputy to UN special envoy Kai Eide, told the Washington Post that Secretary-General Ban Ki-moon's final instruction before firing him last week was, "Do not talk".
He initially agreed to the request after being given assurances that his dismissal would be presented as a dispute over how the UN mission was handling electoral fraud in Afghanistan. But he decided to break his silence after officials announced instead that his firing was in the "best interests of the mission".
"I might have tolerated even this last act of dishonesty in a dispute dating back many months if the stakes were not so high," he told the paper.
"But the fraud was a fact that the United Nations had to acknowledge or risk losing its credibility with the many Afghans who did not support President Hamid Karzai," he added.
Galbraith also launched an outspoken attack on Eide, accusing him of repeatedly denying or downplaying vote rigging that favoured Hamid Karzai.
Eide had told Galbraith not to pursue allegations of fraud and had sided against him when he demanded Karzai-appointed electoral officials enforce anti-fraud measures.
Galbraith, who oversaw UN support to the elections, said his colleagues were now so disgusted by the events that led to his firing that at least five were planning to resign.
Galbraith said, "As many as 30 per cent of Karzai's votes were fraudulent, and lesser fraud was committed on behalf of other candidates.
"The fraud has handed the Taliban its greatest strategic victory in eight years of fighting the United States and its Afghan partners," he added.