Key, a former investment banker, said he supported a merit-based appointment to replace Dominique Strauss-Kahn as managing director of the International Monetary Fund, after Strauss-Kahn resigned to face sexual assault charges.
Noting Lagarde was widely considered the front-runner, Key said she had the credentials for the position.
"I actually met her when I went over on my trip to Paris a couple of weeks ago. She's super impressive I've got to say," he told TVNZ.
Emerging economic powers such as China, India and Brazil are calling for an end to Europe's 65-year monopolisation of the Washington-based IMF's top job.
Under a longstanding deal between Europe and the United States, a European has always held the top IMF job while an American leads the World Bank.
Frenchman Strauss-Kahn resigned last Thursday, five days after being arrested in New York on charges of sexually assaulting a hotel maid. He has denied all charges.
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