Having lost the toss when they eventually started at 2pm, the Jacob Smith-captained Bay skittled the hosts for 99 runs in 53 overs.
Son Angus Schaw took a five-wicket bag and worked clinically again in tandem with fellow spinner Ajaz Patel.
"In the first innings Jazz and Gus bowled 30-odd overs between them to bowl them out," said the senior Schaw.
The Bay seamers had toiled and were mostly unplayable, beating the bat for frugality.
The visitors then declared at 5-100 from 27.1 overs to ensure first-innings points were in the bag before trying to beat the elements again when play resumed at 2pm yesterday after more water had crept under the covers.
Smith and his troops then proceeded to march towards an outright win, skittling Horowhenua Kapiti for just 39 runs in 20.2 overs with seamers Kurt Richards and Blair Tickner hard to get away as they came up with the figures of 3-9 and 1-12, respectively.
"Our catching behind the wicket was extremely good so that was good to see," said Schaw, saluting elder son and wicketkeeper Scott Schaw for making a good fist of things in trying conditions that offered "bounce and turn".
The Bay were mindful that Manawatu and Taranaki were heading towards outright victories as well.
They play Wairarapa in Masterton on December 5-6 in the next qualifier.