Mostly, the couple prefer anonymous giving - something they've done throughout the country over the past 10 years.
"We don't stand up for anything unless we think there is a wider benefit."
Mr Plowman hopes that talking about the Rotoroa gift will encourage others to give.
He says that as well as protecting the privacy of his family, the reason for keeping out of sight is the country's tall poppy syndrome.
"There are a lot of other people interested in doing the right thing in helping the community or the country, but as soon as they put their head up to comment, to get buy-in to the debate, the criticism comes out and that's not comfortable," says Mr Plowman.
The Plowmans advocate "practical" philanthropy, especially endowment funds that provide income in perpetuity, and become involved in big projects only where they feel they can make a difference.
"Rotoroa Island was one that we call 'the green theme' - having a park or place people just pleasantly enjoy with no pressure. It's a place for people to visit, have leisure time, enjoy the beaches and the water and the birds."
The Plowmans negotiated a 100-year lease with the island's owners, the Salvation Army, which until 2005 had used the island for drug and alcohol rehabilitation work.
The lease, which was paid up front, continued an association going back to 1973 when Neal's father Jack gave $1 million to the organisation.
"We had some very senior people working in the company, including Cyril Coulstock, who were staunch Salvationists. We were able to monitor and evaluate their integrity first-hand," says Mr Plowman.
The family made its fortune from a laundry business, founded in 1910, that became New Zealand Towel Services and was sold to an American company in 1998.
Mr Plowman believes everybody who has enough disposable income should invest in some philanthropic activity.
"In the US, it's normal for businesses to put at least 10 per cent of their capital wealth into endowment funds for the future of everybody - not just their family or business."
