"We will be back," she said at National's function in Auckland on Saturday night.

National, which governed for nine years under the leadership of John Key and Bill English until Jacinda Ardern took power in 2017, was destroyed at the ballot box.

After polling 44 per cent in 2017 - well ahead of Labour - the party's vote collapsed to just 27 per cent this year, with 81 per cent of the vote counted.

Ms Collins' caucus will fall from 54 members to around 35, with generationally blue seats turning red in the Labour landslide.

That is likely to include deputy leader Gerry Brownlee.

National has held his seat of Ilam and its previous incarnations since 1948.

"It is an outstanding result for the Labour party (and) it has been a tough campaign," Ms Collins said, revealing she had phoned Ms Ardern to concede.

The 61-year-old was the fourth opposition leader for the Nationals during the three-year term.

The party turned to Ms Collins in July after Todd Muller resigned due to mental health after less than two months in the role.

Collins' grip on the leadership will come under huge scrutiny in the wake of the loss.

She declined to stand down, saying "we will take time to reflect and we will review and we will change".

"We will re-emerge from this loss a stronger more disciplined and more connected party," she said.

"I promise you, the National party will be a robust opposition. We will hold the government to account for failed promises and we will push on behalf of all New Zealanders.

"Three years will be gone in a blink of an eye ... just to avoid any doubt, tonight is the start of the next campaign. Bring on 2023!"

Australian Associated Press