Batting first, Joey Yovich led the way for City with 69 off 45 balls, including 10 fours and a six.
Contributions from Horner (35), Harry Darkins (25) and a seven-ball 24 from Ian Page took City to 172/3.
Maungakaramea's innings was essentially a one-man show as Tyler Lortan scored 89 off 75 balls to try take his side to the win.
But no one other than Mark Southee (21) made it into double figures as Maungakaramea fell 32 runs short.
Meanwhile, Onerahi looked to gain revenge after losing by three runs to WBHS the previous week but were in early strife after a top order collapse.
Cullen Lowe (3-11) was the chief destroyer as he took three of the first four wickets, leaving Onerahi reeling at just 45-4.
The fourth wicket brought Fletcher Coutts (39 off 28) to the crease and, combined with Henry Cooper (51 off 48), put on 66 for the fifth wicket, which steered their side out of strife and to a total of 124-6 from their 20 overs.
WBHS' innings never really took off as new ball pair Scott Robertson (3-11) and Tom Herman (2-10) scuppered any chance of a successful chase.
Rory Christopherson (29) and Caleb Boswell-Smith (20) fought hard, but the rising run rate and a lack of wickets in hand spelled defeat for WBHS as they fell 40 runs short.
Onerahi then cruised to a nine-wicket win over Kaipara Flats after dismissing them for just 79.
They lost a wicket early but Cooper and Brad Kneebone saw them home.
In other results, Matthew Taylor's 4-8 took Maungakaramea to a 45-run win over Kaipara Flats and WBHS beat Kamo by eight wickets.