Wanganui started much better in the second half with a penalty by Mark Davis and a brilliant try by Davis again off a 70m break by Wanganui's exciting new "state of origin" player Saul Chase.
That gave the home side an 18-10 buffer, and Davis added to that with a 20-minute penalty. But only the pace of local wing Simon Dibben stopped another individualistic try for Horowhenua when replacement wing Mike Tambour intercepted - that would have bought it to within a point with 11 minutes left.
The score was boosted with a Chase try in the final minute after fine work by himself and replacement wing Nic Harding - but really good moments from Wanganui were few and far between.
A recurrence of an ankle injury by Manawatu loan prop Gene Ropoama and the return of powerful Fijian No 8 Nemani Negusa on Monday further weakens the Wanganui pack for next Saturday's big match with Poverty Bay.
Best of the home side were Nagusa and loose forward/lock Fraser Hammond, while lock Jon Smyth's workrate was back to its best.
Aucklander Rhema Sagote was strong in midfield and replacement halfback Matthew Koubaridis and Chase added sting later on - but it was an average performance by Wanganui.
Horowhenua's scrum was clearly superior, with loosehead Aaron Kearney dominant, No 7 and captain Antony Fox excellent and lock Ryan Shelford very strong.
But Horowhenua's problem was in creating tries, as it was last week. Their tries came from Wanganui mistakes expertly taken by loan centre Perry Hayman.
Wanganui 28 (Tremaine Gilbert, Saul Chase, Mark Davis tries; Davis 3 pens, 2 cons) Horowhenua-Kapiti 10 (Perry Hayman 2 tries). HT: 8-10.