Dill continued to be a thorn in Onerahi's side as he ripped through the middle order to leave them reeling at 100 for 8 before the rain set in.
Dill ended day one with figures of 16.1-5-6-21, leaving Onerahi with a lot of work to do on day two.
Meanwhile Whangarei Boys High School were in a fairly strong position against Westech Automotives Maungakaramea.
WBHS batted first and started strongly through Cullen Lowe, Luke Trigg and Kian Bird.
Lowe and Trigg put on 54 for the opening stand as they weathered some handy opening bowling before Trigg fell for 19.
Lowe and Bird then combined to put on another half-century partnership as Lowe in particular scored quickly.
Maungakaramea made a vital breakthrough as Lowe fell for 70, an innings which included four boundaries and a trio of sixes.
The introduction of Mark Southee to the bowling crease swung momentum in Maungakaramea's favour as he grabbed three quick wickets (3 for 10) before WBHS declared at 154 for 5.
In reply, Maungakaramea were 27 for 2.
Meanwhile Duracrete Products City have made their way to the reserve grade T20 final after a rain-affected win over WRMK Kerikeri.
City batted first and got off to a flier courtesy of Joey Yovich, who belted 53 off just 25 runs inside the first six overs.
Kerikeri made crucial breakthroughs in the middle order as leg-spinner Max Hart (2 for 17) hit a good length.
Brutal late hitting from Simon Fenwick (70 off 36, with seven boundaries and four sixes) took City through to 176 for 5 from their overs.
In reply, Kerikeri got through to 43 for 4 from eight overs before the rain set in.
City took the result by 36 runs on the Duckworth Lewis system.
The other semifinal between Bream Bay and Kamo High was abandoned due to rain.