Waving the innings off after 77 overs, after scoring at a good run rate above four-an-over, Marist's bowlers wanted a decent crack at United over the rest of the day's play.
The O'Leary brothers – Connor and Shaun (2-24) – delivered to pick up both openers quickly at 6-2 by the fifth over.
However, Shaun O'Leary's fellow Whanganui Collegiate and Wanganui representative player Carter Hobbs (40 not out) was joined by former Wanganui skipper Simon Badger (37) to dig United out of the hole, hitting seven boundaries between them.
Badger gave up a catch to Stewart off Shaun O'Leary not long before the close of play which could yet prove costly, as Sharrock joined Hobbs to see out the afternoon with the match delicately poised - United on 88-3 needing a further 234 for first-innings points.
Day 2 will be played on November 20.
In the other derby fixtures of the opening round, the Coastal Challenge Twenty20 winners Levin Old Boys already have first-innings points after surpassing Weraroa CC's first innings at Weraroa Domain, while Paraparaumu Medical Centre CC had an embarrassing follow-on and still have not surpassed Kāpiti Old Boys' low score on a day for the bowlers at Donnelly Park.
Follow results and draws for the Coastal Challenge 2-day tournament at https://www.crichq.com/competitions/7265/draws/51826/rounds
Scoreboards for November 6
Coastal Challenge 2-day
Marist 1st innings 322-9 dec (C Stewart 99, N Harding 97, M Fraser 70no; H Morrison 4-55). United 1st innings 88-3 (C Hobbs 40no, S Badger 37; S O'Leary 2-24).
Kāpiti Old Boys 1st Innings 153 (D Browne 38, R Hickey 37, T Harris 24no; S Windle 4-25, D Reder 4-34). Paraparaumu 1st Innings 107 (J Horrell 28, B Gill 27; C Andrews 4-47, Z Benton 3-23, D Franks 2-13). Paraparaumu 2nd Innings 34-1 (D Browne 12no).
Weraroa 1st Innings 155 (L Spring 43, J Marshall 39, K Maclachlan 33; R Taylor 5-26). Levin Old Boys 1st Innings 187-6 (M Good 76no, D Sanson 35, R Taylor 26; H Heald 2-22, K Fahey 2-28, J Dawson 2-45).
United CC vs Burger King Red Star: unknown.
P2 40 Over
Wanganui United 3rds 98 (Z Guiniven 33; K Watson 3-25, R Verma 2-12, C Meredith 2-14) lost to David Jones Motors 2nd XI 102-5 (Z Small 16, C Meredith 16; J Whiteman 4-33) by five wickets.
Marist Bs 96 (K Watkin 53; J Healy 3-40, D Maraki 2-15) lost to Tech Old Boys 99-0 (B Hunter 55no, D Reuters 37) by ten wickets.
Kaitoke 96 (R Bellis 18; D Ford 3-5, A Pond 2-6, F Lane 2-28) lost to Matt Burke Engineering Marton Saracens 98-1 (T Westwood 68no, A Spence 23) by nine wickets.
Wicket Warriors Whanganui 163 (V Chacko 43, L Cherian 28; S Abernethy 4-42, J Allpress 2-5, O McVerry 2-31) lost to Whanganui Collegiate Senior Black 165-3 (C Rees 67no, T O'Leary 54no) by seven wickets.
Whanganui Collegiate Senior Blue 140 (L Brown 50, O Mabin 20; M Deighton 3-14, Roger Moore 3-28, J Donaldson 2-40) lost to Wanganui Renegades141-5 (B Heap 32no, R Moore 28; O Mabin 2-19) by five wickets.