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The 63-year-old Waimate vet has just shifted offices for the first time since he arrived in town in 1977.
The Leeston lad, straight out of veterinary science studies at Massey University, landed a job at what was then the Waimate Farmers Veterinary Club.
It had two vets, the ''senior'', Lindsay Knight, only three years older than Mr McLachlan.
Sheep and beef farms dominated the workload.
''There were no deer farms. We had no lectures or education with regard to deer.''
As different types of farming became popular, he learned as he went. Some of the fads he saw off were mohair goats, ostriches, fitches (''posh stoats''), and rabbits.
In the early 1980s, vet practices were under pressure to breed up livestock through embryo transfers - another skill to acquire.
''Everybody appreciated the fact we were all still learning. Perhaps expectations were not as high as they are now.
''There were no computers or cellphones. We had radio telephones. We were on the same frequency as the St John ambulance service and the fire service, as well as 40 farmers and lots of businesses.''
The vets tended pets as well as farm animals. Mr McLachlan taught himself small-animal orthopaedics, taking satisfaction in ''rebuilding broken dogs''.
''We were jacks of all trades and masters of some.
''I'm a No 8 wire bloke. I can think things out laterally and problem-solve.''
Border dyke irrigation in the Morven-Glenavy scheme revolutionised coastal farming, Mr McLachlan said. Land-owners could grow enough grass to feed more sheep, then increased water volumes allowed for dairy cows.
There were about 6000 cows in the district when he arrived, half a dozen herds providing town-supply milk. The dairy sector now accounted for about 60% of the workload for the vet practice, which has eight vets.
It had become Waimate Vet Services in 1989, then amalgamated with Oamaru Veterinary Services in 2007.
Mr McLachlan, who has dropped back to four days a week, said his job was ''hugely different'' from when he started. It entailed a lot more preventive work, planning and record-keeping.
He likes talking to farmers about the old days, and mentoring new vets.
''I'm the oldest in this practice by 20 years but it doesn't stop me getting together with them socially.''
Five of the vets, including him, took part in the Waimate Theatre Company's recent Heavens Above show.
''It was good fun. That spills over into the workplace, that social enjoyment.''
Playing many sports and being a singer and bagpiper helped to integrate him into the community. Marrying a local woman, Kristine Bleeker, made an even stronger connection.
Mr McLachlan played the bagpipes at the Edinburgh Military Tattoo in 2003 with the City of Dunedin band and has performed in Switzerland, at Windsor Castle, and in Russia.