But Peters downplayed his role, echoing Ardern's comments about the lack of a mandate from the public.
"The Prime Minister said, clearly, she discerned that there wasn't sufficient support in the country," Peters said.
"That's the political reality we have to face."
Peters brushed off questions about whether NZ First held too much power.
"Politics and coalitions are difficult to operate. This has been a most successful coalition in an unexpected way. It's not a matter of being happy or who won or who lost.
"What really matters is: have we got the right policy - all of us - that has the support of what I believe is the mass majority of New Zealanders?"
He said the $2 million that will be spent on the Tax Working Group was worth it because the majority of its recommendations will be actioned.
Those included vacant land taxes, multinationals including Facebook paying more tax, and providing tax incentives to improve productivity and not consumption.
Peters has opposed a CGT in the past and he repeated that again today.
"It was its absolute complexity, and I've never seen anywhere in the world where the stated purpose for which it was introduced ever ended up being successful.
"We decided there are better ways we might reform our tax system to give people a fairer go and where the gap between the rich and the poor may be closed, but this is not the pathway."
Ardern said there had been no horse-trading between Labour and NZ First over the CGT, and asked whether NZ First owed the Labour party something after today's announcement, Peters said: "The Labour Party is in Government because of my party."
Green Party co-leader James Shaw said he was disappointed with today's announcement, but he pledged to keep working with Labour to reform the tax system.
Shaw had previously said that the Government did not deserve to be re-elected if it backed away from a CGT, and today he defended those comments.
"When I said that I was making a call for bold action, and when you look at what this Government is doing on climate change, on cleaning up our rivers, on ending homelessness, we are a bold Government.
"I absolutely believe we deserve to be re-elected."
He said it was the nature of MMP that one party could not call all the shots.
"I understand the nature of the Government, and there are a number of decisions that are made where not everyone gets what they want.
"But if I had to choose between this Government, which has done more in the last 18 months than the National Government did in the last nine years, I would choose this Government every single time."
He did not think NZ First had a disproportionate amount of power in the coalition, saying all three parties had to agree for major decisions.
