Westbury wrapped up first innings points by mid-afternoon as veteran duo Matthew Battle and Nick Spencer had a day to remember at Launceston.
Sharing the new ball on a green NTCA No.1 deck, Spencer (5-41) took his first five-wicket haul since returning from a five-year break while Battle (3-28) also took his best figures of the campaign.
The pair reduced Launceston to 9-78 before Matthew Woods (44) and Jakeb Morris (11 not out) helped the home side post 123.
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"Today's one of those days that happens pretty rarely when most things seem to go to plan," Westbury captain Dan Murfet said.
"The wicket was doing a fair bit and Spence and Beej really reaped the rewards of that. Spence had it nipping around all over the place and I certainly wouldn't have wanted to be batting.
"He knows what he's doing with it and when he's up and about he's one of the best bowlers in the competition."
Battle then backed up his bowling feats with the willow, putting on 96 for the first wicket with Dom Barrett (36) before the latter was stumped off Cody Townend.
The former Shamrocks coach enjoyed another 50-run stand with Murfet before launching the final ball of a Ben Humphrey over into the carpark to reach 100.
Humphrey (2-22) got his revenge with an LBW next over while Tom Gray dismissed Matty Allen off the last ball of the day to have the Shamrocks 5-218.
"It was one of those sort of wickets that we thought 120 could potentially be a competitive score - there was still enough in the wicket but thankfully Beej and Dom got us off to a really good start," Murfet said.
"[Battle] has been batting really well - he made 70 last week against South, that was probably his best knock this year and then he's bettered that again today. He's a beautiful player, we're really lucky to have him."
ELSEWHERE IN SPORT
There was another ton at Invermay Park as Nathan Philip registered back-to-back centuries to lead SOUTH LAUNCESTON to 8-339 declared.
Needing just a boundary to reach three figures, Philip was given out caught behind before a conferral between the two umpires saw the left-hander called back to the crease.
The 26-year-old reached three figures in the next over and then went into overdrive, amassing 10 fours and four sixes before being caught and bowled by Jono Jones (3-48) for 128.
The knock came as part of a 141-run seventh-wicket partnership with Danish all-rounder Anders Bulow (84 off 110 balls), who posted his highest Cricket North score.
"We didn't control it for the whole day but we really cashed in from the first drinks break of the second session," said South skipper Tom Waller, who was celebrating his birthday on the same day as middle-order batsman Jeremy Jackson.
"We've known [Bulow] has got the capability to bat - it's just taken him a little while to get the pace of the wicket," Waller said.
"He plays a lot on concrete back in Denmark so it's been a really good experience for him to play regularly on turf.
"He's never played two-day cricket before this year and it was really good from him today to get 84 - he was close there at the end to get his 100 but he did the team thing and tried to get some runs so that was great."
Mowbray saw off a tricky nine overs to finish 0-21 with John LeFevre on 11 and Luke Scott 10.
- Nathan Philip 128
- Matthew Battle 100
- Anders Bulow 84
- Nick Spencer 5-41
- Matthew Battle 3-28
- Jono Jones 3-48