Rotoroa Island in the Hauraki Gulf will open to the public on Saturday for the first time in more than 100 years.
The Salvation Army bought the island in 1907 and for nearly a century had a drug and alcohol rehabilitation centre there.
The centre closed in 2005, a 99-year lease from the Salvation Army was negotiated in 2008, and the Rotoroa Island Trust was formed.
Trustee and project manager John Gow says visitors will be able to explore the heart of the old centre's community, which includes a jail, a chapel and a schoolhouse.
He says the island will also appeal to nature lovers, as more than 300,000 native plants have been reintroduced to it as part of a conservation project, and another 150,000 are still to be planted.
"This is the beginning of the next hundred years," Mr Gow says, "and that makes it very special."
Rotoroa can be reached by ferry from downtown Auckland.