The move creates New Zealand's largest market research consultancy and is seen as key to improving TNS's reach in the region.
David Lowden, TNS chief executive, said: "Conversa Global has established a strong presence in the New Zealand market through its ability to deliver added value, via what we believe to be a unique combination of commercial and market information experience.
"The combination of TNS and Conversa global will offer services and insights not currently available in the New Zealand market."
Conversa Global's revenue for the year ending March 31, 2007 was £5.6m, with gross assets valued at around £300,000.