Negotiations had been taking place over the past two weeks.
"We needed to obtain additional supplies," Ardern said at today's media briefing.
There are 21 new community cases today, all in Auckland, Director-general of health Ashley Bloomfield said.
There’s one additional case which is yet to be classified as either a community case or a border case. This is being investigated.
Ardern said vaccine demand had been at 180 percent recently. Per capita the rollout had been higher than the peaks in places like Canada and the UK.
To keep up with this surge demand the Government had been negotiating with Pfizer and other countries to increase supply.
Additional supply was being finalised for September, Ardern said. Final contracts were still being signed so she could not finalise countries nor quantities, but they were Pfizer vaccines.
This would also allow New Zealand to continue its vaccine sharing arrangements with Pacific nations.
"To say the negotiations had been highly complicated would be an understatement," Ardern said.
The total number of unlinked cases had fallen from 33 to 24 since yesterday.
Of yesterday's 20 cases, 17 were known contacts and 16 household contacts already isolating.
Just four had been in the community, creating eight exposure events.
"Clearly all the numbers are moving in the right direction," Bloomfield said.
There were six people still in ICU, including four on ventilation.
Ardern said there remained enough vaccines in the country to keep up with surge demand. Extra supplies had been secured from multiple countries and more details could be revealed in "a matter of days".
Ardern said she would not and could not reveal the amount of money paid for the new vaccines.
"They require purchase agreements in both directions. One country purchases and then the other country purchases back."
Ardern said 682,000 vaccines were in stock, the vast majority at sites already nationwide.
Ardern has been tight-lipped on who she's been speaking to about receiving more vaccine supply.
But she has powerful friends all over the world, among them Canada's Justin Trudeau, France's Emmanuel Macron, Germany's Angela Merkel, Denmark's Mette Frederiksen, and Spain's Pedro Sánchez Pérez-Castejón.
These countries all have between 60 percent (France) and 73 percent (Denmark) of their populations fully vaccinated, whereas just over a quarter of New Zealanders are fully vaccinated.
And the transtasman heat is on, with Australia having secured quick supplies from Poland, Singapore, and - as announced this weekend - the UK.
Today's update comes as the country outside Auckland prepares to move to Delta level 2 - including more mask-wearing and tighter restrictions on gatherings - from midnight tonight, following no sign of any cases outside the City of Sails.
Auckland remains at alert level 4.
Cabinet will meet on Monday next week to reassess the alert levels for the whole country, including Auckland.
Yesterday was the third straight day of 20 new cases, but the number of cases that were infectious in the community has been declining from 27 on Wednesday, 17 on Thursday, eight on Friday, six on Saturday, and five on Sunday.
The number of mystery cases yesterday was 33.
