Finance Minister Simon Hamilton believes so many public sector workers could rush for a voluntary redundancy scheme that it could run for at least three years.
The DUP MLA predicted: "There will be more than we will be able to afford in the first year."
He envisages that the exit scheme could also cost more than the planned £100m.
Addressing the finance and personnel committee at Parliament Buildings yesterday, he said: "I think in reality it will probably cost more... and I think it will go into further years as well."
Mr Hamilton explained that the impact on public sector services would be too great to allow everyone eligible to leave within a one-year period, therefore it would have to be graduated over two or three years or "even more".
Although he was unable to put an exact figure on the number of looming job losses, he warned there would be "considerable" numbers.
Mr Hamilton told committee members that with a 1.6% reduction in Stormont's budget in 2015/16, which equates to £213m of savings, "we have to take some very difficult decisions and there are tough choices ahead".
He revealed that "significant work" has already been done behind the scenes in terms of the workforce restructure, which unions have claimed will equate to thousands of posts being axed.
"We have the beginning of a credible plan in terms of workforce restructuring."
The Finance Minister said the voluntary exit scheme would be in place in a "very short period of time".
He continued: "I will present it very shortly to the Executive I hope in the form of a reform and restructuring plan."
However, when asked about the exact number of people that would be leaving the public sector, Mr Hamilton stated that "the fine detail is not there yet" but added that there "was no targeting of particular grades".
"It's not as simple as how much money have you, how many people can you remove, who are the first hundred or thousand people who put their hands up."