Prince William, wife Kate and baby Prince George arrived in New Zealand yesterday and were greeted by Key at Wellington Airport before meeting the conservative leader again at an official welcoming ceremony.
He is scheduled to meet the royals at least five more times during the 10-day tour, prompting opposition Labour Party leader David Cunliffe to say they were being used for "pre-election PR from the prime minister".
"I guess he likes the camera time," said Cunliffe, who will attempt to derail Key's bid for a third term in the election on September 20.
"I think the public will watch closely to see how the prime minister treats his presence on the tour in an election year."
Cunliffe said he welcomed the young royals, who are extremely popular in New Zealand, but said contacts with politicians should be "as even-handed as possible" so close to an election.
Key dismissed suggestions he was trying to get too close to the royals, saying all national leaders had to meet visiting dignitaries regularly.
"I'm not going to be barnacle attached to them over a 10-day tour. There's a degree of normality about my engagements," he told reporters yesterday.
Opinion polls show Key appears headed for a comfortable victory in September's election, although New Zealand's complicated system of proportional representation makes it notoriously difficult to predict results.
The royals were enjoying a rest day in Wellington on Tuesday as they recover from the long flight from London for the three-week tour of New Zealand and Australia.
Prince George, who was the star of the show when the family arrived on Monday, will have a play session with a group of local babies tomorrow. His parents will ride in a jet boat and sail America's Cup yachts while in New Zealand.
The Australian leg of the trip includes an excursion to Uluru (Ayers Rock) in the country's fabled red centre.