Mr Duthie said the "entrepreneurial" Alec came to South Otago with his brother-in-law James Smith, initially splitting 50 acres (20ha) of Tokoiti farmland between them, which soon expanded.
Going on to thrive in several areas of agriculture and other business, the families and their descendants - including an additional line surnamed Martin - became well established.
He calculated the "happy accident" of Alec's early settlement in the region meant he now bore distant relation to as many as a third of all inhabitants from Dunedin southwards.
"The idea for a family reunion sprang up in 2014, which would have been the 200th anniversary of Alec's birth in Kirriemuir, Scotland.
However, it's taken a few years to get the history in order and plan a gathering, so we're only really under way now."
Although the family knew of many Duthie and related branches, he suspected there would be others as yet undiscovered.
"We're not holding the reunion until next Easter, but now's the time to set the wheels in motion and alert those who might be interested in attending. We're also seeking additional historical materials to fill in a few blanks in the family story."
Duthie relations were now scattered from the far north to the tip of the south, and he hoped media exposure for the reunion might help assemble as many as 200 visitors for the four-day reunion.
Forebear Alec had begun his New Zealand life as an assistant to noted Otago surveyor Charles Kettle, and had had a hand in many of the instrumental early building projects in an embryonic Dunedin.
That included the main street of Port Chalmers, to where the reunion would return.
"We'll divide our time between Port Chalmers, Dunedin, and Milton and South Otago, with a strong emphasis on family history, as it overlaps with the story of Otago."
Mr Duthie said his research into Alec's life had led him down many interesting paths, intersecting with a key period in Otago's formation.
"If you know where you come from, it helps you understand where you are today. I'm looking forward to sharing that story with others."
Details of the reunion can be obtained on Facebook, or by emailing ahduth@slingshot.co.nz.