A tech commentator says Spark's decision to block the controversial website 8chan goes beyond its jurisdiction.
8chan is currently offline, after its hosting company terminated it as a customer.
Spark has announced if 8chan finds another host provider, it would block access.
The 8chan internet forum was used by the accused Christchurch mosque gunman to distribute his manifesto and livestream the attack.
8chan hosts message groups and has been attacked for disseminating hate speech from white supremacist groups and inciting violence.
Spark said the government should step in and technology commentator Paul Brislen agreed.
"It's very, very nearly the edge of what's acceptable for what your internet provider to be doing in this kind of situation," he said.
After the shootings in Christchurch, the ISPs moved swiftly to take down access to any of the manifesto or any other content being put forward by the Far Right, that was appropriate. This is probably still appropriate given 8Chan's record, he said.
"I'm as uncomfortable as they [Spark] are about it. They do really need to find a new way to manage hate-speech and extremist content on the internet," he said.
He said the situation was an unusual one for Spark - a commercial entity - to find itself in.
"It's much like the Telecom of old to decide which phone calls you can and can't make," he said.
In a statement Spark called for the government's help.
"Appropriate agencies of government should put in place a robust policy framework to address the important issues surrounding such material being distributed online and freely available.
"But in the absence right now of such a policy framework . . . we believe the right thing to do right now is for us to block access to 8chan."
Mr Breslin said the risk was someone would now turn around and say 'okay you blocked 8Chan because of hate speech, now I want you to block this other website because it allows people to access something else.' It might be hate speech, it might be pornography, it might be something that speaks out against a religious group or ethnicity.
"You start down a certain track of Spark or any of the other ISPs being forced to decide what is and isn't acceptable for the NZ public and that's not their job at all. They really shouldn't be doing that.
"That's the job for the public of New Zealand to decide this is acceptable, this is not, these are the people who are responsible that these are the standards and these are reinforced through our laws." he says.
He said the government needed to make a decision about who should be regulating.
"The government should be mandating some entity, presumably the chief censor's office, to oversee this kind of content. Having your ISP, a commercial body, to decide what you can and can't watch, is really not ideal. In the ideal world we would have a nationwide standard and this would be applicable, you could appeal, there'd be a process.
"None of that's in place at the moment. In effect, we're relying on Spark and the other ISPs to do the right thing, and that's always a very difficult spot for any company to find themselves in," Mr Breslin said.