True, there's a few other "ISIS archives" that are used largely by intelligence analysts, journalists, historians, and academics for study. But I don't think NZ is saying "Hey journalists and academics, don't study a dump of 8chan!"

What NZ has been saying is that there should be no way for an ordinary, unaccredited member of the public to view any archived copies of any of the Christchurch killer's propaganda anywhere online, something normal internet users apparently can do with ISIS propaganda via archive.org. They've been quite consistent about this since day zero, from what I can tell.

I wouldn't be so sure, following the aftermath of Christchurch my network (AU) and several others have not unblocked the Chan's. I don't believe they are planning to either.

Can you clarify if there are actual legal restrictions on that content in place, or is this still unilateral ISP action?

There are no current legal restrictions as I understand, it is the decision of my provider to continue restricting this access. The restrictions are not unilateral, the largest ISP has released the restrictions.

I will be changing providers as soon as my contract allows.

No, they're just trying to be the sole group who decides who is part of those two groups.