During the past two weeks, 138 of the endangered birds have been relocated to Five Fingers Peninsula/Tau Moana, from predator-free Breaksea, Anchor, Chalky and Passage Islands where populations of the birds had previously been established.
Five Fingers Peninsula, about 3300ha, adjoins Resolution Island/Mauikatau which at 20,888ha is Fiordland's largest island.
The project was led by the Fiordland Conservation Trust in partnership with private donors, Te Rununga o Oraka Aparima, Te Kaitiaki Roopu o Murihuku and the Department of Conservation.
Doc biodiversity principal ranger Lindsay Wilson, of Te Anau, said the project could be a ''game changer'' for saddlebacks.
''It's a large site with ideal habitat so the population could increase by thousands more birds.
''This translocation will also show us whether species that are vulnerable to stoat predation can establish in areas with low predator numbers.
''This information is vital as the drive towards Predator Free 2050 sees increasing predator control across the country.''
Additionally, the project could pave the way for other predator-sensitive species, including kakapo, to be returned to Resolution Island.
Fiordland Conservation Trust chairman Kim Hollows said the trust was proud to support the saddleback transfer.
''The birds will be monitored to see how they do, and whether they breed and disperse to the wider island.''
A network of nearly 3000 traps kept stoats at near-zero levels on Five Fingers Peninsula and very low levels on Resolution Island.
The entire peninsula was surveyed by a team of stoat detection dogs before the release and no evidence of stoats was found.
In the past 50 years, South Island saddlebacks (tieke) had recovered from a low of about 36 birds to nearly 3000 on predator-free islands, but population growth has stalled due to the islands reaching carrying capacity.
Yellowhead (mohua) and brown teal (pateke) have been successfully returned to Resolution Island in recent years.
The island is also home to rock wren (piwauwau) and southern kiwi (tokoeka).
