The bail announced Tuesday was part of a deal between the government and the Bangladesh National Party, which Zia, 63, heads.
In a similar move three months ago, the court released former Prime Minister Skeikh Hasina, leader of the Awami League party and a political rival of Zia. Hasina was released so she could get medical treatment.
The release of the two women or the "Begums" clears the way for the country's two major political parties to participate in general elections expected by year-end, the Voice of America reported.
The report said the interim government's leaders had previously hoped to end political graft in the country by arresting the two women but later it became evident that without them there could be no fair and peaceful elections.
"They must lift the (state of) emergency and announce the schedule of the election. During emergency, no fair election can be held," a leader of Zia's party told VOA.