Christchurch will also host nine games, including the second semifinal.
The Government has allocated $5.5 million to help fund the tournament and expects a decent return given it will be broadcast to a television audience estimated at 170 million in more than 100 countries and will see 90,000 visitor nights spread across the seven cities.
"I think that's money well spent and is an important building block to allow the event to be hosted in New Zealand and to ensure it's a success," Key said.
"One of the things the Rugby World Cup taught us was that these events are much more than just sport."
New Zealand has previously hosted the boys' (1999) and girls' (2008) under-17 World Cups and the under-20 tournament is the biggest this country can expect to stage. It also comes in the same year as New Zealand co-host the Cricket World Cup.
New Zealand are the only team who are so far guaranteed a place at the tournament as hosts and have played at three of the last four tournaments.
The Junior All Whites have never progressed to the second round but an under-20 side will play in this year's ASB Premiership in the hope of improving that record.