This is on top of the $30 million-plus the Plowmans have spent funding a 99-year lease from the Salvation Army to create a conservation park and restore historical features of the former alcohol and drug rehabilitation centre, including a chapel, jail and school house.
The latest project is part of a transition of healing the body and soul for 100 years to a new century of healing the land, which started in 2008 with cutting down and mulching 20,000 pine trees and planting 400,000 native plants.
Ian Fraser, the zoo's New Zealand fauna curator who will oversee the conservation sanctuary, says it will not be a purist's restoration project that discounts wildlife that had never occupied the island but more of a "show home" with a strong focus on education.
"This will be a great platform to train conservationists of the future," he said.
The project will begin with a survey of the existing birds, reptiles and invertebrates, leading to a 25-year wildlife restoration plan.
Apart from mice, the island is virtually pest-free. Rats have been eliminated in the past year and there is a comprehensive set-up of 120, GPS-tracked bait stations. However, Rotoroa's proximity to Waiheke, Ponui and Pakatoa islands means it will always be susceptible to pests.
The island has a limited number of native wildlife species, such as tui, kingfishers, weka, dotterels and petrels, but Mr Fraser said this would change in the next year or two.
He said the first birds to be relocated to the island could be "relatively robust" kiwi, either brown kiwi or little spotted kiwi. Other birds that could eventually settle are saddleback, whitehead, robin, bellbird and kakariki.
Mr Fraser said the island, with 30ha in grass, could also take the flightless takahe, one of New Zealand's most endangered and popular species.
Other possible wildlife are giant weta and endangered skinks and geckos.
Zoo director Jonathan Wilcken said wildlife would have to be managed carefully in conjunction with other island sanctuaries in the gulf.
"New Zealand leads the world in island restoration and has always shown more appetite for managed conservation," he said.
The zoo plans to establish a school programme in the hope a generation of children will grow up with a sense of ownership in the renewal of Rotoroa Island. About $280,000 of the $4 million gift will be spent on a Rotoroa Island display at the zoo's new $16 million Te Wao Nui New Zealand precinct.
A reptile house and aviary will be built on Rotoroa as part of $1.5 million assessing existing species and introducing new ones. The funding, over five years, provides operational costs for the council-owned zoo at no cost to the ratepayer.
Mr Wilcken said the $4 million birthday gift was an extraordinary act of philanthropy by the trust, which could never have happened otherwise.
The Plowmans, who made their money through New Zealand Towel Services, are an intensely private couple who live in Kerikeri and leave matters to the trust.
Trust chairman Barrie Brown said the public would be able to see active conservation on the island, east of Waiheke Island and a 75-minute ferry ride from downtown Auckland.
Trustee John Gow said creating a sanctuary on Rotoroa Island for endangered wildlife was a natural extension to protecting its conservation and heritage.
Since February 2011, the 82ha island has been open to the public after being off-limits to everyone apart from 1200 recovering alcoholic and drug addicts cared for by the Salvation Army.
When visitors and boaties arrive at Home Bay, they find the remnants of the Salvation Army's treatment centre, including a primitive two-cell jail block, a simple brick chapel containing several craft works made by patients, a restored schoolhouse dating back to the 1860s, a buttery/butchery and a woolshed-style exhibition centre, designed by architect Rick Pearson.
Visitors have a choice of walks, one of which goes from Home Bay, past a wetland to pristine Ladies Bay, where there are changing rooms, toilets and barbecue facilities. From there, it's a relatively easy climb through one of the remaining stands of native trees to a cemetery, surrounded by a white picket fence overlooking a dramatic headland.
At the southern end of the island a dramatic Chris Booth sculpture points out to sea and nearby Ponui Island.
Doorstep sanctuary
*The 82.5ha Rotoroa Island is east of Waiheke Island.
*It and neighbouring Pakatoa Island were used by the Salvation Army as an alcohol and drug rehabilitation centre.
*The army bought Rotoroa in 1908 and public access to the island was banned.
*After catering for thousands of people, the army's centre at Rotoroa closed in 2005.
*A 99-year lease was bought by a trust in 2008.
*The island is being turned into a conservation park, funded by Neal and Annette Plowman.
*The Plowmans, through the trust, are contributing $4 million to create a wildlife sanctuary with Auckland Zoo.
Getting there
360 Discovery operates ferries to the island seven days a week between December 24 and February 6 and five days a week between February 7 and April 28.
360discovery.co.nz
Current wildlife
Weka, New Zealand dotterel, grey-faced petrel, tui, kingfisher, little grey warbler.
Possible new wildlife
Kiwi, whitehead, saddleback, robin, bellbird, kakariki, takahe, giant weta, flax snail, shore skink, moko skink, robust skink, Duvaucel gecko
