Brazil striker Cristiane pounced on a poor clearance late on to break New Zealand hearts with a 1-0 win at the Millennium Stadium, Wales.
The victory meant Brazil, silver medallists at the last two Olympics, made it two wins out of two at London 2012 after thrashing Cameroon 5-0 in their opening game.
On Saturday, New Zealand frustrated Brazil thanks to a disciplined defensive display in the Women's Olympic Tournament Group E encounter and looked set to hold the Samba Queens to a goalless draw.
But stiker Cristiane, who plays her club football with Russian club WFC Rossiyank, calmly lobbed Ferns' goalkeeper Jenny Bindon after her punch went straight to the Brazilian.
"New Zealand are a very good team, very organised," said match-winner Cristiane.
"Nobody said any of these matches at the Olympics were going to be easy."