A Hauraki Gulf island used as an alcohol and drug rehabilitation centre for 100 years is being gifted to Auckland by a couple who made a fortune selling towel supplies.

Philanthropists Neal and Annette Plowman have funded a 99-year lease of Rotoroa Island from the Salvation Army and formed a trust to create a conservation park and restore historical features including a chapel, jail and school house.

When the park opens next February, it will be the first time in 100 years that the public have been allowed on the 82ha island, east of Waiheke Island.

It is hoped Rotoroa will become a tourist drawcard like Tiri Tiri Matangi Island, but with the extra attractions of its colourful history and four beautiful beaches.

The gift, worth many millions of dollars, ranks with the 2006 donation by retired Kaipara farmers Pierre and Jackie Chatelanat of their $10 million Atiu Creek farm overlooking Kaipara Harbour to the Auckland Regional Council and American billionaire Julian Robertson's gift of 15 modern masterpieces worth $115 million to the Auckland Art Gallery.

The Plowmans, a very successful but private business couple, have been reluctant to talk about the gift.

In a brief interview, Mr Plowman said the gift was to give Aucklanders access to the island and continue the family's support for the Salvation Army and many wonderful Salvationists who had worked for his companies here and in Australia.

The Plowmans made their money through New Zealand Towel Services, originally a family laundry founded in 1910. The company grew into New Zealand's largest towel supply and industrial uniforms company, expanded into Australia, and was listed on the New Zealand Stock Exchange. It was sold to an American company in 1998.

These days, Neal and Annette Plowman divide their time between Kerikeri and Australia.

The seven-member trust is chaired by retired PricewaterhouseCoopers partner Barrie Brown and includes high-flying New Zealand executive Chris Liddell, who joined General Motors as chief financial officer from Microsoft last month.

John Gow, who has created the Connells Bay Sculpture Park on Waiheke Island directly opposite Rotoroa Island, is another trustee and project leader.

The trust includes two members from the Salvation Army, which is the sole beneficiary.

Since taking control of the island in February 2008, the trust has cut down and mulched 20,000 pines trees to allow a major revegetation project that will eventually have 400,000 native plants.

About half have already been planted and this year another 90,000 will be planted.

There are also plans this year to build a woolshed-style visitor centre designed by architect Rick Pearson that will tell the story of the Salvation Army's occupation of the island.

The Salvation Army originally owned nearby Pakatoa Island, but when this became too small, it purchased Rotoroa from the Ruthe family in 1908 for £400 and public access was banned. Rotoroa Island was used by male patients and Pakatoa Island by female patients.

The patients' accommodation blocks on Rotoroa have been demolished, but seven staff houses have been kept and three will be available for short-stay rents.

The trust also has plans to sell 10 lifestyle blocks on a licence to occupy for 95 years with a right of renewal for a further 99 years to provide some income. Sculpture and other art projects are planned.

Commissioner Donald Bell, the Salvation Army's territorial commander and a trustee, said the transformation of the island over the past year towards a conservation park had it looking fabulous.

"I wish to express our gratitude to the benefactors who have made this possible as a gift to both the Salvation Army and the public of New Zealand," he said.

Business secretary Major Bruce Vyle said proceeds from the lease have been put into an endowment fund to provide an additional and ongoing revenue for Salvation Army work. The financial details are confidential.