But 8chan still has a Twitter account, despite calls for Twitter to take it down.
Ardern said she and Dorsey had a long conversation about 8chan and similar online platforms, and while Dorsey had "an awareness of the issue", no decisions were taken.
She said one issue was how easily another platform would spring up if 8chan's Twitter handle was shut down.
"It's a much bigger issue than just 8chan."
It was the second one-on-one meeting between Ardern and Dorsey, and Ardern praised Twitter for being particularly engaged in the efforts, led by New Zealand, to tackle online terrorist content.
Dorsey told Newshub that the meeting was "great" and described Ardern as "amazing".
They had met in person in Paris in May in the lead up to the Christchurch Call, an agreement to stop terrorist and violent extremist content that was signed by governments and online platforms.
Ardern said she would report on the call's progress when in New York for the UN General Assembly later this month, where tech companies will also gather.
"None of the momentum since March 15 has been lost, and if anything, they're more resolute now more than ever," Ardern said.
She said she would make an announcement in New York about how tech companies respond to a crisis, which was announced as a key focus of the Global Internet Forum To Counter Terrorism (GIFCT) in July.
The forum was established in 2017 by Facebook, YouTube, Microsoft and Twitter to disrupt the way terrorists use such platforms.
In July the GIFCT said the Christchurch Call was the catalyst to introduce joint protocols for responding quickly to an event such as March 15, which was livestreamed and shared widely on multiple platforms.
"Based on the joint protocols, we will work together to categorise the type of incident and the anticipated level and degree of online impact," the GIFCT said in July.
"We will also set up formal channels of communication so we can share intelligence and content with non-GIFCT companies and other stakeholders, as needed."
In the lead up to the six-month anniversary of the Christchurch shootings, which is on Sunday, Ardern said that more needed to be done to stop bullying and discrimination.
That included stopping it in schools and workplaces, but also ensuring that migrant and refugee communities were getting the same opportunities to participate in society as everyone else.
"The areas where there is more work to be done - and what the Muslim community has really raised with us - is what we do around discrimination," she said.
"We are not a perfect nation. We knew that before the 15th of March, but this has highlighted the experience of many in our community ... and the obligation we all have to address that."
