MacLachlan (24) started with a 76 on Saturday afternoon but followed with two sub par rounds of 71 yesterday to stave off a late challenge from St Clair's Brent McEwan.
McEwan finished two shots behind and Millbrook professional Ben Gallie was two further shots back in third.
MacLachlan played at No 2 for Wellington in 2012 and 2013 with above average results and is trialling to make the Otago team this year.
He moved permanently to Queenstown in February and has been working in his father's timber company in the town since.
The tournament was reduced to 54 holes when Saturday morning play was deemed impossible due to flooding on the putting surfaces.
The tournament began on Saturday afternoon in very strong cold southerly winds which were not conducive to low scoring.
Balmacewen bomber Jason Hughes belied the conditions shooting a par 72 to hold the overnight lead by two shots.
Chisholm Park winner Duncan Croudis (Otago) and McEwan finished on 74 with Gallie a further stroke back.
Brandon Hodgson (Otago), MacLachlan (Millbrook), Connor Ross (St Clair) and Mat Tautari (Southland) were tied for fifth place on 76.
MacLachlan took the lead yesterday morning with a 1 under 71 to head Gallie and McEwan by two shots with 16 year old Inia Logan (Wakatipu Junior) and Croudis one further shot back.
Meanwhile, Hughes dropped back to sixth place four shots off the pace with a 7 over 79 with Chris Snow (St Clair) a further shot back.
Perhaps the honours for the weekend should go to Frankton professional Jamie McIsaac who ran away with the masters (over 40) event with a 4 under 36 hole total of 140.
McIsaac shot a 1 under 71 in atrocious conditions on Saturday to hold an overnight lead of nine shots.
He saved the best for Sunday with a faultless bogey free round of 69 which included a three putt from 6m on the final green for a par five.
Andrew Hobbs (Island Park) posted a par 72 yesterday to add to his 80 on Saturday to be 12 shots back in second place.
Arrowtown's Kevin Clark was third place with two rounds of 80 but he played much better in his second round despite being plagued by three double bogeys.
