New Zealand's richest man Graeme Hart has gone into business with his son Harry and the pair's company has purchased cereal company Hubbard Foods and Gregg's sauces - right after buying Kiwi food company Hansells.

"The bringing together of Hubbards, Hansells and Gregg's Sauces under Walter & Wild combines some of New Zealand's best-known food brands into a food manufacturing powerhouse," said Harry Hart, owner of Walter & Wild.

Graeme Hart is also a shareholder of Walter & Wild, a spokesman confirmed.

"The breadth of expertise, manufacturing capability combined with an understanding of markets, both here and offshore will drive further growth and innovation. We now have a significant brand portfolio reaching consumers from breakfast to dessert," said Harry Hart.

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Brands in the Walter & Wild portfolio include Aunt Betty's, Hansells, Hubbards' muesli and cereals; and Greggs table sauces.

Hubbards founder Dick Hubbard said: "After making the equivalent of 3 billion bowls of mueslis and cereals over 30 years, it is time to hand over the reins to a younger generation. In selecting a buyer, it was important that Hubbards remained in New Zealand ownership. I'm very pleased to have achieved that goal".

"Harry and his team have the energy and enthusiasm to take Hubbards forward. I am really proud of what Diana and I have achieved. The company is in excellent shape with a strong management team, and the brand is a clear market leader in muesli. We had a lot of fun along the way, and I want to thank New Zealand for the opportunity to serve," said Hubbard.

Graeme Hart, New Zealand's first billionaire, added nearly $2 billion to his fortune in 2017, according to the Bloomberg Billionaires Index.

The owner of Auckland-based private equity firm Rank Group saw his net worth jump 21 per cent to NZ$11.03b in the year to December 28.

That's an increase of some $1.92b and makes him the 203rd richest person in the world, according to the index.