NZ's next Prime Minister Jacinda Ardern addresses nation

Labour leader Jacinda Ardern has just secured NZ First's support to form a Government, but planning for that Government is already firmly underway.

She addressed the nation as New Zealand's next Prime Minister just over 90 minutes after NZ First leader Winston Peters announced his decision, and admitted she was as in-the-dark as the rest of the country.

"I joined with New Zealand as we collectively learnt the information together," she said in her press conference.

Despite the short timeframe, she's already announced how the new Government would be shaped.

Ms Ardern confirmed Mr Peters has been offered the position of Deputy Prime Minister and NZ First has been offered four Cabinet positions and one Parliamentary undersecretary.

Mr Peters said in his own press conference earlier in the evening he hadn't yet decided whether to accept the Deputy Prime Minister position, which Ms Ardern reiterated.

Meanwhile the Green Party "will have ministerial portfolios", she also confirmed, and she said they were trying to be as "proportional as possible".

Mr Peters had notably campaigned on a number of "bottom-line" policies he said would be non-negotiable for any parties who wanted to form a Government with NZ First.

Some of those bottom-lines and NZ First's other policies were different from Labour's campaign promises, however Ms Ardern said there weren't any dramatic concessions.

"I only made concessions in areas that the New Zealand Labour Party would feel comfortable with and aligned with our values," she said.

"I am very comfortable that we have focused on the end outcomes that Labour has always wanted to achieve... On each of the principals we've campaigned on, we're sticking to."

The weeks of negotiations between Labour and NZ First - and NZ First's own separate negotiations with National, with whom it could have also formed a Government - were "dominated by discussion around policy", Ms Ardern said.

"It formed a solid foundation on which we will now build a coalition Government," she said.

When it came to those policies, she was particularly firm on the matter of re-entering Pike River Mine.

"Absolutely," she confirmed.

Ms Ardern also confirmed they would "prioritise" a ban on foreign buyers for property in the country.