Jacinda Ardern will become New Zealand's third female prime minister and the youngest leader in more than a century.

Less than three months ago, Jacinda Ardern wasn't even leader of the New Zealand Labour Party — but the 37-year-old transformed the election campaign with what was called the 'Jacinda Effect'.

However, that enthusiasm wasn't enough to form government and the Labour Party finished second in last month's election.

That result suggested the National Party incumbent Bill English would hold onto power by doing a deal with kingmaker MP Winston Peters and his New Zealand First Party.

But against the odds, yesterday Winston Peters threw his support behind Labour.