Mark Lezar also had a good match, taking 2/27.
Leading the charge to set Onerahi's first innings total of 187 were Junaid Khan (58), Mark Lezar (38) and Tom Herman who remained on 38 not out at the end of Onerahi's 20 overs.
In an earlier match on Saturday, Onerahi hosted City in the first of their two T20 matches for the day.
Onerahi batted first but were all out for 99 with City's Lucky Singh taking 3/16.
Onerahi's Dillon Lees described Singh as "the standout bowler" of the first innings.
City looked to make light work chasing Onerahi's low score but couldn't make the required inroads against Onerahi's bowlers who dismissed City for 99.
In the instance of the two sides being drawn on 99, a super-over was brought into play with both sides having to provide one bowler, three batsmen and outscore each other to take the overall win within a restricted two over format.
City batted first but only managed to compile two runs off the two allowed overs.
Onerahi's Mark Lezar then took on the minute total headfirst, bringing up the win with a four and thus giving Onerahi their first win of the day.
In other matches over the weekend, Kaipara hosted two T20's at Bourne Domain.
They scored a win off a total of 143 in their first match versus Maungakaramea.
Kaipara also managed to notch up a win in their second T20 against Whangarei Boy's High School (WBHS) making it two wins from two for the day.
Kaipara reached the end of the first innings against WBHS all out for 173. WBHS took to the crease with a slow batting innings but only managed 92 runs.
