Sawang Boonmee, its secretary general, told reporters at 2pm that the mistake was detected about an hour after early voting started.
Officials at the Nonthaburi central voting station immediately isolated the 100 envelopes so that they could be corrected.
Sawang assured the 100 voters that their votes would not be wasted and they would be sent to the correct constituencies after officials change the codes.
Sawang explained that the officials put postal codes instead of provincial codes on the first 100 envelopes. After the mistake was detected, the officials immediately isolated the box with the wrong codes so that the error could be fixed.
“The envelopes will be mailed to the correct constituencies,” Sawang said.
He said social media posts that claimed Nonthaburi election officials got the postal codes wrong were false, explaining that provincial and constituency codes were required but local election officials had put postal codes on the line where the provincial code was supposed to be placed.
This will not happen on election day, because no mailing is required, Sawang said, explaining that voters will go to polling stations in their constituencies on May 14.