So when he dined on oysters, whitebait and lamb with Mr Key and his family at Premier House last night, William was not allowed anywhere near the kitchen.
Instead, the meal Mr Key described as "quintessentially Kiwi" was prepared by the experts - again led by Ruth Pretty.
The guest list was also a lot shorter. In 2010, the "state barbecue" was attended by the well-connected and famous, including Richie McCaw and Hayley Westenra. This time round, it was the tight five - the royals, himself, his wife Bronagh and son Max, who met William during the Keys' stay at Balmoral last year.
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Despite the public figures involved and the public purse paying for it, Mr Key and his office were tightlipped about the dinner until yesterday, saying it was "private".
Mr Key confirmed yesterday it was paid for by the taxpayer and when asked whether that meant it was not such a private dinner after all, he said "well, it kind of is and it isn't".
He said the royal couple had requested something informal in their programme and a "catch-up".
He had originally intended to host the royals at his Parnell home, but because they were based in Wellington to be close to Prince George that had not suited their programme.
"They were certainly keen to catch up, they were certainly keen to have something that was relaxed and in a fun environment."
He did provide more details, saying the menu would "showcase New Zealand" and included Bluff oysters and whitebait entrees, followed by a main course of either rack of lamb or salmon. Pudding will be pavlova and hokey pokey icecream.
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Mr Key expected the royal couple to be at Premier House for a few hours - but doubted Prince George would be there.
"There is obviously a welcome invitation for him but I'm pretty sure it will be past his bedtime."
It has become a bit of a tradition for Mr Key and William to cook for each other: after Mr Key hosted the 2010 barbecue, William returned serve by helping out cooking a barbecue for the Key family at Balmoral.
Mr Key went one up in the ledger again last night.
In the lead-up to the trip, a London-based Maori suggested in the British press that the royals might be offered kereru to dine on.
That was not on the menu, but Mr Key's comments did prompt one wit on Twitter to ponder what the recipe for "Quintessential Kiwi" was.
Dinner at John's
The guests: Prince William, Catherine; John, Bronagh and Max Key.
The place: Premier House, Wellington.
The menu
Appetisers: Bluff oysters and West Coast whitebait.
Main: Rack of lamb or snapper.
Dessert: Pavlova and hokey pokey icecream.
