Alhambra-Union started and finished the stronger to grind out a hard-earned win in the drizzle at Watson Park.
Alhambra-Union scored three tries to one as it gained its fifth win in seven games, while Harbour suffered its second consecutive home loss.
Alhambra-Union scored two tries to lead 12-0 after nine minutes but, despite dominating territory and possession, it could not add to its tally during the remainder of the spell.
Harbour scored through No 8 David Kahukiwa just before the break and, as it camped deep in the Alhambra-Union half for much of the second spell, an upset was a distinct possibility.
But Peter Breen made the game safe seven minutes from the end when he charged down a kick from Johnny Legg, regathered and raced 30m for a try which he converted himself.
Props Scott Manson and Dane Maraki were powerhouses in the Alhambra-Union pack and lock and captain Matt Kissick had an excellent all-round game. Breen controlled play well, but little was seen of the Alhambra-Union outside backs.
Harbour recovered well to be competitive after a disastrous start but it, too, was plagued by errors and wrong options on attack.
Peter Mirrielees had a strong game at hooker and Kahukiwa was energetic in general play and off the back of the scrum. Lance Taylor played tidily at first five-eighth but the outside backs had few chances.
- Alhambra-Union 19 (Peter Breen, Matt Kissick, Scott Manson tries; Breen 2 con), Harbour 10 (David Kahukiwa try; Lance Taylor con, pen) . Halftime: Alhambra-Union 12-10. Referee: Doug Rodgers.