Peters praised supporters, candidates and their families for never losing the faith.
NZ First's North Shore candidate and former mayor Andrew Williams said they had always been confident they would make the 5 per cent threshold.
"We want to be a party that represents all New Zealanders. We don't see ourselves as left or right or any particular leaning," he said.
Williams said he didn't see any dangers that NZ First would cause uncertainty in the financial markets.
"We want to bring more certainty to the NZ economy."
Many at last night's election night party were scathing of mainstream polls which had placed NZ First as low as one per cent.
NZ First activist Lee Caton, 19, said the result was "bloody fantastic".
He leaflet-dropped thousands of homes in the Maungakiekie electorate and convinced four teenage friends to vote for NZ First.
Voters spurned Prime Minister John Key's pleas earlier in the week and voted Peters back in.
Paul Douglas, chairman of the North Shore electorate, said internal polls had shown 7 to 10 per cent support for the last 12 months.
"Winston's strength and the pace that he has been working at for the last 12 months has been unbelievable," said Douglas.
"There's been nothing in the media - they think we are all old folks and were prepared to ignore us.
"But take a look around the room here and you are not seeing old folks.
"I think the media have seriously missed the boat and haven't been watching or haven't seen what's been going on."
Written off by many, attacked in the final days of the campaign by all parties on the right, most notably by Key, Peters bounced back.
And last night he was predictably unpredictable. He arrived at the Spencer Hotel and a gave a five-minute speech to supporters, firing barbs at the media and veiled shots at his doubters, before rushing off to "attend other meetings".
"I want to thank those voters around the country from Kaitaia to Invercargill, and from from further north and south than that ... who brought us back home to Parliament," said Peters.
He hinted at being unfairly hamstrung to the point that affected the party vote.
"Dare I say I regret that we haven't brought in more [MPs], and there are circumstances for that, and I'll talk about that in the next few days.
"Tonight it's about thanking the people who have shown they believe in democracy still and they believe in a fair go," he said.
"Others had huge resources. Some parties spent more than four thousand times what New Zealand First spent on this campaign."
Peters was not shy of a few words for the media.
"We'll never forget the nature of this campaign. For much of it New Zealand First was marginalised, stigmatised and even demonised in a substantial black-out from my friends in front of me in the media.
"But nevertheless it speaks of the enormous character of the New Zealand First membership that they never stopped believing that we could come back."
He said the party would be "positive and constructive" as it headed for the opposition benches.
"There are serious economic problems we face as a country and we will never get through them unless we're united as one people.
"In that unity we share in the pain and we all share in the gain and that's what New Zealand First stands for," said Peters.
He congratulated the other NZ First candidates that will be MPs.
"I wish I could be with you, but you didn't hire a big enough hall," Peters told supporters.
The result sees a bevy of NZ First candidates entering Parliament, including former Williams, Rodney local board member Tracey Martin, and former Investigate columnist Richard Prosser.
Peters has said he will sit on the opposition benches, but that did not mean New Zealand First could support confidence and supply votes for a Budget it approves of, or vote in favour of Government bills. It would vote issue by issue, he said.
