The knock was Ronchi's seventh first-class hundred and showed Wellingtonians there is plenty to like about the Dannevirke-born player, who is likely to come in to the thoughts of the national selectors once he is able to represent New Zealand.
Elliot, who has struggled for form this summer, closed the day unbeaten on 145 in what was a much-needed confidence booster.
The 32-year-old, who played a handful of tests for New Zealand and 37 one-day games plus a solitary Twenty20 match, belted 14 fours in his 267-ball knock that was his seventh first-class century.
Worryingly for the New Zealand side, CD leg-spinner Tarun Nethula bowled 21 wicketless overs for 101 runs and struggled to make an impact.
Zimbabwe international Kyle Jarvis, who is contracted to CD for the remainder of the summer to cover the pace-bowling injury crisis, picked up 3-78 from 19 overs and was the pick of the bowlers for the visitors.
At Colin Maiden Park in Auckland, the home side won the toss and put Northern Districts in to bat. ND amassed 266-6 before bad light stopped play at 5.04pm.
Recalled international Daniel Flynn finished the day unbeaten on 68, while Brad Wilson also made 54 and Hamish Marshall contributed 49.
Flynn's knock would have pleased national coach John Wright who has indicated the gritty left-hander may bat at the top of the order for New Zealand against South Africa at the Basin Reserve in the third test, which starts on Friday.
Top-of-the-table Northern are looking to sew up the Plunket Shield title with a big performance and Auckland seamers Mitchell McClenaghan (2-52) and Michael Bates (2-35) combined to take four wickets, while Bruce Martin claimed the other two.
In Dunedin, only 13.2 overs of play was possible on day one of the match between Canterbury and Otago.
The visitors batted first and were 36-2 with openers Peter Fulton (10) and George Worker (1) both dismissed when the rain came and the players left the field.
Ian Butler (1-3) and James McMillan (1-15) each claimed one wicket for Otago.
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