The 62 alleged South African mercenaries in Zimbabwe were only expected to arrive in the country on Thursday, their lawyer Alwyn Griebenow said on Wednesday.
"If everything goes according to plan we will leave Zimbabwe at nine tomorrow (Thursday) morning," he said from Harare.
He said the men would travel by bus and would enter the country at the Beit Bridge border post.
The group will be released from Chikurubi maximum security prison outside Harare later on Wednesday.
They would then be taken to immigration officials who would complete their deportation documents, Griebenow said.
He said the men would be transferred to Harare Remand Prison where they would spend the night.
Zimbabwe's chief immigration officer is personally handling the matter.
Under Zimbabwean laws, foreigners fined more than 200 Zimbabwean dollars (R0,21) have to be deported once the fine was paid and the sentence served.
"So obviously they are being seen as prohibited immigrants," Griebenow said.
The men were supposed to have been released on Tuesday as their sentences had expired. They were convicted of breaching Zimbabwe's aviation, immigration, firearms and security laws.
Griebenow said on Tuesday he had only managed to see four of them as prison officials only allowed him to meet two of them at a time.
"I don't know why they decided this. It's never been like this before.
"They are physically and mentally drained. The lack of assurances and guarantees is really getting to them," he said.
The group has spent over 12 months in prison. They are still awaiting the outcome of an appeal lodged by Zimbabwe's attorney-general, Sobuza Gula-Ndebele, against a reduction of their sentences which would have allowed them to be released in early March.
Zimbabwean court officials confirmed on March 2 that the men were scheduled for immediate release after a successful appeal to the high court for a reduction of their sentences.
A week later, with all the paperwork completed, their lawyer and families waited in vain for their return which was delayed when Gula-Ndebele filed an application to appeal against the court's decision.
He argued that early release only applied to Zimbabweans.
Griebenow said the appeal would only be of value to two pilots who had received 16-month sentences for aviation and immigration-related offences.
Two of the men due for release on Wednesday - Francisco Marcus and Melane Moyodue - are ill with tuberculosis believed to have been picked up in prison.
Accusations of mistreatment of the prisoners have surfaced during their imprisonment, with Griebenow saying their living conditions were "horrible".
Their prison food had little nutritional value, they slept on the floor, and sometimes weeks went by without running water, he said.
The group was arrested at Harare International Airport when they apparently landed to refuel and pick up military equipment.
Zimbabwean authorities said they were on their way to join 15 other alleged mercenaries - including eight South Africans - arrested in Equatorial Guinea around the same time.
The group in Equatorial Guinea were convicted and given long prison sentences, in what has been called a flawed trial, for attempting to overthrow the country's long-time dictator, Teodoro Obiang Nguema. - Sapa