Ardern's popularity soared earlier this year after she led a successful effort to halt the spread of the virus by implementing a strict lockdown in late March.
The 75-year-old who first entered parliament 41 years ago, refused to say his long and colourful political career was over.
With a quarter of the vote still to be counted, Labour has 49 percent of the vote and its ally the Green Party has 7.6 percent, with Collins' National Party holding 27 percent.
She displayed both empathy and decisive action on gun control after a white supremacist gunman killed 51 Muslim worshippers in the March 2019 Christchurch mosques attack.
New Zealanders will also vote on whether to legalise marijuana and euthanasia.
Ardern cast her vote in her Auckland district Saturday after visiting a market in the heavily Polynesian south Auckland suburb of Otara, where she was thronged by supporters.
In the first televised debate before the vote, Ardern said she stood by a virus programme that has limited the outbreak's toll to 25 deaths in a population of five million.
The centre-left leader, who is riding high in opinion polls, also acknowledged concerns from rivals that curbs on campaigning would unfairly weigh the election in favour of her government.
With 1.5 million people under stay-at-home orders, and millions more at risk of a wider outbreak, Ardern said she was seeking advice on delaying the September 19 election.