However, she made clear she had not given up hope of forming a Government.
"I simply cannot predict at this point what decisions other leaders will make," Ardern said, adding Labour would do everything it could to work with any party that shared its values and aspirations for a better country.
Ardern attended the funeral of Colin Meads during the campaign, and in her speech cited one of his quotes.
"If you come off the field and feel you haven't done enough. You've let the side down...I know we gave it our all.
"No matter where tonight's result takes us, I am committed to a future...that is better. So let's keep doing this."
After her speech Arden faced a media throng alongside her partner Clarke Gayford, who held a large bunch of flowers.
She had only spoken to English, but said that was not a concession phone call.
"I think in an MMP environment he wouldn't have expected that either. We both acknowledged that what we have had tonight does require further conversations."
Asked if there was an outside chance of Labour forming a government, Ardern said much of that would be determined by "people beyond myself - I'm not the sole determinant of that so I certainly wouldn't want to make a judgement on that at this point".
Ardern, who took to the stage to a song by Shapeshifter's Stars, said she would contact other parties in the morning.
Asked if she was disappointed with the result, Ardern said she was a perfectionist. However, Labour's vote had lifted in the past seven weeks and she was proud of her team.
On what issues would be important to Labour in any coalition negotiations Ardern said she had made her view clear on the need to clean up rivers, but those conversations weren't for tonight.
At the start of the night the mood at the Labour event was of nervous excitement, with the main room bathed in red and two camera booms swinging over the seating. A DJ played hits from Kelis and the Black Seeds, while supporters posed for photos with dress-up gear at a special photo booth.
Labour Party president Nigel Haworth was full of praise for Ardern.
"She has been absolutely magnificent. She stepped up at very short notice. I don't think she has put a foot wrong. She has dealt with difficult issues promptly and very well indeed. I am extraordinarily proud of her and what she stands for in our party."
At 8pm Gareth Morgan was on the big screens calling a "landslide" victory for National. Two glum-looking supporters emptied their wines at 8.45pm as Newshub host Patrick Gower came on screen talking about the "huge" vote so far for National at 46 per cent.
Later in the night with the numbers still the same, boos rang out as Paula Bennett came on screen, with the feed quickly changed to Radio NZ. The biggest cheers came whenever the situation in Waiariki was mentioned, with Labour's Tamati Coffey ahead of Maori Party co-leader Te Ururoa Flavell.
Ardern received a huge reception when she entered. Haworth introduced her and said, "Labour is back".
When Ardern made it to the stage she said when she took on the leadership seven weeks ago she made the comment that being Opposition leader was the worst job in politics.
"I was wrong. The last seven weeks have been nothing but a privilege."
