The best chance of the opening 45 minutes fell to Rosie White who neatly brought down a high cross into the area before turning and firing just over the Portugal crossbar.
At the other end, goalkeeper Rebecca Rolls - who started the match in place of number one goalkeeper Erin Nayler - had little to do but the New Zealand 'keeper was thankful just after the half hour mark as a sharp turn in the area produced a shooting chance for the hosts which was dragged wide.
Early in the second period the Football Ferns survived a Portugal shout for a penalty after defender Rebekah Stott collided with an attacker in the penalty area but the referee waved play on.
The second half continued as the first had unfolded with regular sights of goal tough to come by for both sides until the 69th minute when the New Zealand side broke quickly downfield.
A cross came in from the left flank and Longo arrived late to turn it goal wards over the leap of both Amber Hearn and the Portugal goalkeeper to confirm the result for Readings' team. The final margin might have been 2-0 but Sarah Gregorius' well-taken stoppage time header was ruled out for offside.
Algarve Cup 2016 - Group B
Portugal 0-1 New Zealand (half: 0-0)
NZL: Annalie Longo 69'