It also lost a number of key electorates, including the former safe seats Ilam, Nelson and Auckland Central.
Asked if Simon Bridges could launch a challenge, Collins said "you'd have to ask him", but she was focused on delivering stability.
Collins said it was "very hard" to see MPs lose their seats, and a big wake-up call for the caucus to focus on what matters to people, and "not talk about ourselves".
She likened her task taking over as leader as a "hospital pass".
"I have absolutely worked my socks off."
Collins said her message to caucus colleagues was the focus needed to now be on 2023.
"The right thing to do is to provide stability for the party," she said after being asked if she should stand down.
When asked if Simon Bridges would have produced a better result, Collins said "let's wait for the review".
Collins said it was a "very difficult" time to take on National's leadership, but it was the right thing for her to do.
Did ill-discipline by MPs hurt the campaign? Collins said most people worked very hard.
"It's not easy to win an election against a first-term government ... you also put the Covid situation onto it ... we did everything we could do."
"The fact we had three leaders in four months was challenging."
On Brownlee, Collins said he had been a great support and any retirement decision was up to him.
"I am here for the party. I am happy to do my bit. And I am very happy doing the job."
Internal polling "was not quite as bad as this result".
Before the second lockdown National was about 39 or 40 per cent, she said.
Denise Lee's leaked email cost "about 5 points", Collins claimed.
"That is not for me to deal with. The review will deal with it," Collins said, after being asked who leaked the email - which was critical of Collins - to media.
Collins said National put out an incredible amount of policy, she said.
"I think a lot of it actually came down to Covid 19. People were frightened about that ... we saw that when the second lockdown came."
Collins said she had no regrets about taking over the leadership, despite it being a very hard time to take it on.
"I gave it my all ... obviously it is very disappointing. I hate seeing good MPs lose their jobs."
The party would conduct the review, not her, Collins said.
"The party itself - we have taken this knock, but you know what, we're down but we're not out."
Collins ended the press conference on that statement, saying she was looking forward to the rugby later today.
Speaking to supporters at National's headquarters last night, Collins promised her supporters "we will be back".
"The National Party will ... take time to reflect and we will review and we will change.
National will re-emerge from this loss a stronger, disciplined and more connected party."
She congratulated Prime Minister Jacinda Ardern for what she called an "outstanding result" for Labour".
"We always knew it was going to be tough," she said.
Collins did not stop to talk to media last night so this morning will be the first opportunity for reporters to question her on what happens next.
But speaking to Q&A this morning, National's campaign chair – and senior MP – Gerry Brownlee said he had talked to Collins and she expressed a desire to "stick around".
"At the moment, Judith Collins is the leader of the National Party and has the support of the caucus."
His future with the party, however, is not as clear.
Asked about his future this morning, he said that was something for him to think about over the next few days.
"I'm going to have a think about things over the next few days."
Meanwhile, Rodney MP Mark Mitchell, whose name has been tossed about as a potential leadership contender this morning "absolutely" ruled out challenging Collins for the leadership.
"That's not on the table, it's the furthest thing from my mind".
