Copp grew up in West Melton, near Christchurch, but is based in New South Wales.
Copp, who has also set records in New Zealand and Australia, shore 523 and du Plessis shore 447.
The South African took up shearing full-time about 16 years ago after retiring from rugby with a fractured skull.
There was no previous mark for the category, which is one of more than 40 overseen by the World Sheep Shearing Records Society.
It was the first world shearing record attempt in South Africa since 2006 and featured four referees from South Africa and Australia.
The last world shearing record in South Africa was the solo and two-stand Merino lambs marks set by South African bladeshearing pair Samuel Juba and Bangali Joel in Victoria West, Northern Cape Province, in February 2006.
The only other record on the continent was the three-stand machines tally of 1208 Merino lambs in eight hours near Tromsburg, about 180km north of Steynsburg, in February 2003.
Here, Ken Norman, who learnt to shear in New Zealand and later settled with his family farming near Pahīatua, shore 456.