Surgeon and wife get the top price for 2015 as they sell a slice of paradise.

A landmark Herne Bay home has sold for $24 million - the most expensive house sale of the year, and all without an official real estate listing.

Commercial property investor Ben Cook has bought the Sentinel Rd property, with its huge grey minimalist three-level house, from endoscopic surgeon John Dunn and his wife, Rose.

Mr Cook, owner of Cook Property Group, is currently in Britain for the Rugby World Cup. He was a shareholder and director of health business Nature Bee, but said he had sold out about a decade ago to found his own real estate business.

The Herne Bay property was unusual, Mr Cook said, indicating the price tag was justified.

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"A lot of people don't realise that the Dunns' house is zoned for 10 house sites and that is why the land value is so high. This is a bit unique, compared to most high-value homes.

"I have no plans to alter/develop the property. When you consider that land is over 4000sq m and can accommodate 10 waterfront homes, the price is a lot easier to understand."

The eye-popping sale price comes as Finance Minister Bill English warned that the housing market in Auckland - where the median house price is now about nine times the average income - remains a risk to the country's economy.

The private off-market Herne Bay sale was by top agent Graham Wall, who also sold the country's most expensive house: the $39 million Paritai Drive property, partly financed by former Hanover Finance boss Mark Hotchin, that Oravida founder Deyi Shi bought in 2013.

The property Mr Cook has bought is on a 4032sq m site and is listed as being worth $16 million ($10 million for the land, $6 million for the house) for rating purposes. The rates are $40,208 a year.

It features six bedrooms, including a separate two-bedroom apartment, a helipad, beach access and a home movie theatre.

John Dunn's brothers include the Catholic Bishop of Auckland, Patrick Dunn, and City Sales chief Martin Dunn, who specialises in selling inner-city apartments.

John and Rose Dunn are significant art patrons along with the likes of Sir Rod and Lady Gillian Deane, Sir Eion and Lady Jan Edgar, Dame Jenny Gibbs, Fran and Geoff Ricketts and Sir James Wallace.

The Dunns have art throughout the house, which was built about five years ago.

The couple's 2007 purchase of the property sparked the sale of their Remuera house in Arney Rd to an Auckland businessman and his family for $8.1 million.

They paid precisely the same amount for what was then a derelict Herne Bay structure. For decades, the site was home to an abandoned concrete shell, once planned to be a grand house.

Before builders shifted in, the Dunns invited some of the city's top graffiti artists to "bomb" the derelict shell on the site.

Mr Dunn was the first New Zealand surgeon to perform a laparoscopic cholecystectomy, in 1990, and more recently he developed dedicated laparoscopic gastric banding for obesity sufferers.

He was a founder of Endoscopy Auckland in Gillies Ave, Epsom.

The house near Home Bay Reserve faces north towards Watchman Island and the Harbour Bridge and is across the water from the Chelsea Sugar Refinery. It stands in an area once nicknamed the Herne Bay Sultanate after the Sultan of Brunei bought houses there.

Yesterday, the Finance Minister, speaking at a social housing conference in Wellington, said the Government would come under huge political pressure to provide assistance to middle-income families if interest rates rose.

"As households get a bit stretched in meeting the rising cost of housing, it inevitably creates pressure for Government to lift the cash transfer programmes," Mr English said.

He said that in the mid-2000s, interest rates of 8 to 9 per cent meant the Government had to compensate by making big increases in Working for Families subsidies and by introducing interest-free student loans.

The Government would rather "get the market right" than have to provide more assistance to families.