John Key has spent half an hour with David Cameron this morning, as Britain's outgoing Prime Minister was packing his possessions into boxes and saying goodbye to Downing Street staff.

Home Secretary Theresa May will be sworn as Britain's new Prime Minister on Thursday morning New Zealand time.

Our Prime Minister is likely to be the last world leader to meet Mr Cameron before he leaves number 10.

John Key told Leighton Smith David Cameron is disappointed - but not bitter at the way the Brexit vote has ended for Britain - and himself.

"It's the brutality of politics, which is when you're out you're really out and notwithstanding that he'd obviously resigned, it was very much unexpected that it would be today."

Mr Key said he's had a long, relatively close relationship with David Cameron - but has met Theresa May on only a couple of occasions.

LISTEN ABOVE: Prime Minister John Key speaks with Leighton Smith