There have been multiple claims of fraud in the August 20 poll, which according to preliminary results, was won by incumbent Hamid Karzai.
The Afghan Electoral Complaints Commission (ECC) and the Independent Election Commission (IEC) are investigating a sample of suspected fraudulent ballots.
"It has been claimed that there was 30 per cent fraud. There is no way to know at this stage what the level of fraud is," Eide said.
"No one knows. I can only say there was widespread fraud."
The final results of the investigations - that Eide said were supported by the UN - are expected in the coming days.
Galbraith was sacked on September 30 by Ban Ki-Moon, the UN secretary-general, due to a public dispute over how to deal with the fraud allegations, which threatened the credibility of the Afghan government and the international strategy to fight the Taliban.
Eide, who was joined by the US, UK, German and French ambassadors at the news conference, said Galbraith's allegations against him "have not only been personal attacks against me and my personal integrity but have affected the whole election process."
He denied the allegations by Galbraith that he had prevented information on high levels of fraud to be disseminated, even to ambassadors in the country.
Eide said that the UN's role "is to support the process, not influence the outcome".
"Some of these allegations were based on private conversation whilst he [Galbraith] was a guest in my house.
"My view is that private conversations around a dinner table in my house remain just that, private."
Karzai received 55 per cent of the preliminary results, with his closest challenger Abdullah Abdullah, a former foreign minister, on 28 per cent.
EU election monitors have said that about 1.5m votes - 1.1m for Karzai - were suspicious.
If no candidate receives at least 50 per cent of the official results, then a run-off between the two top candidates will be held.
However, concern has been raised by authorities that unless results are released soon Afghanistan's harsh winter could make a run-off vote impossible on logistic and security grounds.