Hardcore had not lobbied her and she had not received any communications about the case before it was in the public domain, he said.
Ardern said she received a number of messages from the public and while she would consider releasing them under the Official Information Act if required, she would be reluctant to given the sheer number.
Asked why she hadn't revealed text message previously, Ardern said she was asked only whether she knew anyone who made representations on the Sroubek case.
"I knew nothing of this case until it was in the public domain. I do not know who made representations on the case and I could not say hand on heart whether anyone who contacted me made representations on this case."
National has slammed the Prime Minister after the revelation, with leader Simon Bridges saying Ardern had not been upfront about the message from Hardcore and it was time she told the whole story.
"She's only told us this much because of our relentless questioning. It beggars belief to say that this would be the first contact that she has had with Richie Hardcore about this case."
Ardern said the claims from the Opposition felt a bit "desperate".
Bridges said yesterday Ardern should release the full text message, and asked why Hardcore would have sent a text if she didn't know who Sroubek was.
Instagram photos show Hardcore – a former martial arts champion-turned-personal trainer and now social justice campaigner – posing with Sroubek, formally known as Jan Antolik.
In one photo of the pair, Hardcore wrote in the caption: "My bro fromer #WorldChampion Jan Antolik and I after #training number 11 for the week."
In another photo of the two together, dated May 2013, Hardcore wrote the caption: "Crazy people in the Czech Republic! Big Jan, Joey y Yo! Tough guys do #selfies."
Three other Instagram photos from Hardcore's account show him posing with Ardern.
In a September 2014 photo, he called Ardern "one of the smartest, coolest & compassionate people I'm lucky enough to call a friend."
In another – taken in Parliament in May 2015 – he referred to Ardern as "the people's champ".
The revelation follows weeks of pressure from National MPs about who might have tried to influence Immigration Minister Iain Lees-Galloway over Sroubek, hinting that it might be someone associated with the Government.
Lees-Galloway initially granted Sroubek residency, but has since overturned it.
During Question Time yesterday, National's immigration spokesman Michael Woodhouse asked Lees-Galloway if Hardcore had supported Sroubek.
Lees-Galloway said he would not be divulging who had advocated for Sroubek, saying there was no public interest in the information.
But after Question Time a spokesman for Ardern revealed the existence of the text message to the Prime Minister.
Hardcore is a fan of Ardern, writing on his Facebook page when she was elected Prime Minister: "I'm so happy someone I truly know is a kind and decent person is leading the country".
After the Immigration NZ review and intense political pressure, Lees-Galloway issued a new deportation notice to Sroubek, meaning he would have to leave New Zealand after finishing his jail sentence.
Sroubek is appealing against the notice to the Immigration & Protection Tribunal