New Zealand Prime Minister John Key could become one of the first world leaders to meet the newest member of the royal family when he meets the Queen this weekend.

Mr Key, his wife Bronagh and children Stephanie and Max have been invited to join Queen Elizabeth and her family at Balmoral Castle in Scotland this weekend, during the visit to the UK.

There is speculation Prince William, his wife Catherine, Duchess of Cambridge, and their four-week-old son Prince George will stick around for his visit, after taking a private jet to Balmoral earlier this week.

Their private holiday is the first time Prince George has met his 92-year-old great-grandfather Prince Philip, who has been recuperating from surgery.

Ahead of his own visit, Mr Key was tight-lipped about who the other guests might be.

"Who the Queen might invite to Balmoral is a matter for the royal family - they make the decisions and they also announce who their guests are or aren't," he said.

Mr Key says the visit is a unique opportunity for his family.

"It'll be a tremendous privilege to be there."

He laughed off a question about whether he'd packed a Kiwi-made Buzzy Bee toy in his luggage, in case he needs a gift for the baby, saying he has brought gifts for the Queen, but won't reveal what those are before she gets to unwrap them.