Survival rates of the 20 pateke released last March into the Arthur Valley area of the Fiordland National park had been the highest of all first-time captive releases.
Department of Conservation Te Anau biodiversity ranger Andrew Smart said the birds had made themselves at home, with two pairs attempting to breed, sitting on nests due to hatch "any day".
If successful, they will be the first ducklings produced at any release site within the first year of being transferred there.
The release of 40 more birds was possible because of a nation-wide effort of private breeders, conservation trusts, Ngati Wai, Ngai Tahu and businesses supporting the project.
The project was sponsored by Ultimate Hikes, which runs guided walks on the Milford and Routeburn Tracks, with Real Journeys contributing to flying costs.
"With only around 2000 purebred pateke living in a wild state in New Zealand, this second release of pateke into the South Island is going to make a real difference."
If survival rates were good, more than 200 pateke could be released into Fiordland over the next five years, Mr Smart said.
