The first batch of senators after the new constitution takes effect must be endorsed within 150 days and they will be in office for three years, said a charter drafter.
Gen Lertrat Ratanavanich, spokesman of the Constitution Drafting Committee (CDC), said the rule is included in the provisional chapter of the new charter to ensure a smooth transition.
Under that chapter, of the 200 new senators, 77 will come from elections to be organised by the Election Commission. The 123 others will be selected by a panel to be appointed by the Prayut government.
Gen Lertrat said this "provisional" senate will stay in office for only three years and after that a new senate, which will also be a mixture of elected and selected senators, will take over and remain in office for six years as stipulated in the new charter.
He also defended the CDC's decision to allow the coup-installed government to pick 123 senators.
"The provisional senate has to work with the committees on reforms and reconciliation. So, it makes sense that some of them should come from the interim government.
"If we look at the roles and responsibilities of the senators under the new charter, we see they are focused on screening laws. Their impeachment powers are also limited," Gen Lertrat said.
So far there is no regulation banning the first batch of senators from seeking elections when their three-year term is complete, he said.
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"Let the interim government push reforms and other works forward. An elected government will still have its mandate," the CDC spokesman said.
Gen Lertrat played down criticism the provisional senate was introduced to gain the upper hand when the National Reform Council (NRC) votes on the draft charter next month.
Some critics think the CDC has an agenda to use the provisional senate as a tool to persuade the NRC members to accept the charter.
He said it is too early to tell who the selection panel will choose and noted the panel will seek a diverse senate.
Gen Lertrat also said while the CDC has decided to retain the mixed system for the composition of the senate, it has agreed not to include details of the senate selection in the charter.
He said the details will be addressed in an organic law governing the senate election and selection to be drafted later. The move is intended to avoid growing criticism about the composition of the selection committees.
According to an early draft, the 123 senators would be selected by four selection committees but some critics are concerned the selection panels are not diverse enough.
Meanwhile, a group of NRC members led by Kriangkrai Phumlaojaeng distanced themselves on Tuesday from some of their colleagues who have declared their opposition to the draft charter.
Speaking at a press briefing, Mr Kriangkrai said his group was neither for nor against the draft charter and a decision would be made when it is due. He noted the final version is not completed and slammed those who tried to prematurely decide the charter's fate.
"We'll have to see the final version before we can decide. If it is good for the people, we will vote for it. But if it is flawed, we won't accept it. At this stage it's too early to tell," he said.