They also took out the men's singles and triples, giving them an overall 11-5 lead.
Wanganui manager Alton Heaslip said they hoped to give the visitors "a good hurry up", although the tide began to turn.
"The reality set in, these South Islanders know what they're doing."
South Island won the men's pairs, fours, and drew the women's pairs.
In the second round they came back strongly; winning all but the women's singles to take a 19-13 lead.
Wanganui also came back in the final round to win the men's singles, triples and fours, as well as the women's pairs while drawing the women's fours.
However, the South won the men's pairs, along with the women's singles and triples to stay a step ahead.