Re-runs are necessary because protesters with the People's Democratic Reform Committee obstructed polling in several constituencies on both advance voting and general election day.
EC secretary-general Puchong Nutrawong said the resolution for the poll re-runs had been based on a situation assessment made during the agency's March 7 meeting with governors and election committee chairmen from 15 southern provinces in Songkhla's Hat Yai district.
Under the resolution, a poll re-run will be held on April 5 in Pattani, Narathiwat, Yala, Bangkok and Prachuap Khiri Khan province constituencies where voting was cancelled on Feb 2.
Another poll re-run will be held on April 27 in the constituencies in Chumphon, Nakhon Si Thammarat, Phuket, Satun, Phangnga and Ranong where voting was blocked by PDRC protesters on Jan 26 and Feb 2.
The date for a new election in the 28 southern constituencies where no candidates were able to register for the Feb 2 polls is yet to be set.
The EC said it will wait for a Constitution Court ruling on whether a new royal decree is required to reschedule voting in those constituencies, and whether the government or the EC is authorised to propose it.
On a proposal for the EC to hold the Senate election and the poll re-runs simultaneously on March 30, Mr Puchong said the EC agreed that it was not possible and decided that March 30 voting must be reserved for the Senate election alone.