James Mortimer 01.Apr.2012Getty Images
Tupou scored the opening try of the match, while Alafoti Faosiliva again crossed the line at the beginning of the second half to give the one-time series champions a 12-0 lead.
But New Zealand hit back with tries to Joe Webber and Mark Jackman, forcing the game into extra time, before the flying Tupou crossed to break kiwi hearts.
The result will make the series interesting heading into the final rounds in Glasgow and London, with Samoa currently fifth in the standings with 82 points - and a 19 or 22 point haul (pending the result of the final) could see them make ground on overall series leaders New Zealand who have 111 points.
New Zealand will play England for third place, while Samoa will meet Australia in the Tokyo Sevens Cup Final.