Ardern is expected to make a major health policy announcement about primary care at the launch.
Labour has already committed to an extra $8 billion of health funding in the next term if it gains power.
"Everyone should be able to afford to get the healthcare they need, when they need it," Ardern said in a press release.
As she walked through the market community members stopped to greet the new Labour leader. Supporters are trailing behind with official placards reading "let's do this".
Two university students, selling home-made lotions, said Ardern's presence at the market would invigorate younger voters' interest in the election and bring publicity to the markets.
Stallholder Ma Homg shook Ardern's hand while her colleague filmed the encounter on a phone. Watching the footage afterwards, Homg said: "She's nice."
Afterward one young constituent, sitting atop dad's shoulders, exchanged a round of high fives with Ardern. A serenade by a local singer followed, with Ardern stopping to listen and seeming genuinely mesmerised by the rapper.
Before leaving she gave a speech to a cheering crowd, apologising for intruding on their shopping. "I so appreciate all the warmth and all the support," she said.
"This morning the message is a really simple one. New Zealand is an amazing place, of awesome opportunities. We have the chance to for our kids and our grandkids to have a better life and a better start than the ones before them. We can do better than what we're doing now."
She was concerned about reduced state housing, about the cost of healthcare and rising rents. In the Otara area rents had shot up four times more than wages had, she said.
Her promise of making education free, including wiping school donations, received a big cheer from the crowd.
She ran through several Labour policies, including building more houses, and closing tax loopholes. Low rates of home ownership was "just not good enough".
Afterward, an impressed marketgoer, Karyn Goddard, who had driven from Hillsborough, said she hadn't voted in the last election but was so taken by Ardern, she thought she would this year.
