Department of Conservation (DoC) rangers Theobald, his colleague Paul "Hondy" Hondelink and their pilot Nick Wallis were on their way to undertake tahr control in the Haast area when they died.
Theobald started working out of Twizel in pest eradication in 2010, and since 2015 was based there as a senior DoC ranger.
He joined DoC as a ranger for the Kauri Coast in 1996, after working in various great-outdoors jobs.
Theobald was a world pioneer in the use of dogs to detect predators, starting at Trounson Kauri Park detecting stoats in 1998.
From that early work he developed the National Predator Dog programme in 2000, based in what was then called the Northland Conservancy.
This grew into the nationwide Conservation Dogs Programme which combined Pest Detection Dog and Protected Species programmes.
Theobald first moved to Twizel in a role eradicating pigs, developing a technique using both helicopters and dogs to find the last remaining pigs in areas as big as 10,000ha.
He also focused on a range of species including wallabies, tahr and goats.
His work on pest eradication using dogs took him all over New Zealand, including many off-shore islands, to Japan where the mongoose rather than possums, rats or stoats was the target, and to New Caledonia.
Conservation Minister Eugenie Sage said there was more than 100 years' experience between all three victims of the helicopter crash.
''That loss is huge, because it was real practical skills in the mountain lands and the forests, working with dogs, hunting, that is very difficult to replace,'' she said.
''Scott pioneered the use of conservation dogs which are now used to help keep predators off islands, to sniff out biosecurity risks.''
DoC director general Lou Sanson said Theobald's work had been incredibly helpful for predator control efforts in New Zealand and internationally.
''He went on to help New Caledonia and Japan with mongoose. The predator dogs came from Scott's work and working out how to get dogs to chase stoats and rats.''
Sanson said he expected that after the funerals and farewells a further memorial of some kind would be offered for the three men.