Hassan Sabei said the alliance of Hizb Allah and the Shia Amal had won, by a wide margin, all 17 of the contested seats.
The ticket had already been declared the winner of six other seats in the south where there were no challengers.
Sunday's vote increased Hizb Allah's legislators in south Lebanon from four to five.
The Islamist organisation had already won a seat in Beirut in the first round of Lebanon's staggered elections that began on 29 May.
Message to America
The group hopes its victory will prove its strength and send a message of defiance to the United States amid international pressure on the group to disarm.
Mohammed Fneish, a successful Hizb Allah candidate, attracted the highest number of votes on Sunday, taking 154,056 votes.
His tally surpassed parliamentary Speaker Nabih Berri, the leader of Amal, by about 1000 votes.
The vote across the south was so lopsided that the losers, a range of communists and independents, received little more than one-tenth of the votes taken by the winners.
Among the losers was Anwar Yassin, a communist ex-guerrilla who spent 17 years in Israel jail before he was freed in a prisoner swap last year.
He received 18,450 votes.
The parliamentary elections in Lebanon, which run until 19 June, are the first national polls to be conducted since Syria withdrew all its troops from Lebanon in April.