The Hammers began their Football United Tour with a loss in Auckland, against a Phoenix team that finished ninth in the 10-team A-League last season.
New signings Aaron Cresswell and Cheikhou Kouyate were both named in the starting line-up by Sam Allardyce, with Winston Reid captaining the team in the city of his birth.
Carlton Cole forced an early save out of Phoenix goalkeeper Glen Moss but in the 11th minute, the New Zealand club's skipper Andrew Durante headed an Alex Gorrin free-kick past Jussi Jaaskelainen to give them the lead.
And on 27 minutes, the Phoenix doubled their advantage as Spanish midfielder Gorrin raced through after winning a block tackle with James Collins and finished coolly into the far corner.
Zarate replaced Kouyate on the hour mark and pulled a goal back on 71 minutes, curling a shot inside the post after being set up by Matt Jarvis.
Reid went close with a header in the final stages before Phoenix substitute goalkeeper Lewis Italiano tipped Stewart Downing's goalbound effort round the post in stoppage time.
West Ham now move on to Wellington to face Sydney FC at the Westpac Stadium on Saturday.