"My guess is there's a fairly low-level censor. The mistake from Corrections is not making those letters have to go past someone at a very high level."
He said he had received many letters from inmates, but none that needed authorisation from Corrections' head office.
"I think that will need to happen with this particular guy, and probably should have been happening from the beginning."
Asked if Stevenson should be doing the vetting herself, Williams said that should be considered.
The Howard League, among other things, helped to teach prisoners how to read and write, and Williams said some people will think that the alleged offender should not be able to send any letters at all.
"But we've signed up to international conventions about the rights of prisoners, and certainly they have an absolute right to communicate with their lawyer and family. But beyond that I would be very jumpy about this particular prisoner.
"The content should [also] be censored. We should not allow him to spread his hateful doctrine. That letter should never have got through the system, and that will never happen again."
But Williams defended Stevenson, saying it was "ridiculous" to call for her to resign over the matter.
"The Howard League would not support that. She's very good."
Stevenson has also apologised to Corrections Minister Kelvin Davis, who said that the law will be looked at to see if any holes needed to be plugged.
One of the survivors of the March 15 shootings has spoken out about his heartbreak that the letter was able to be sent.
National Party Corrections spokesman David Bennett said that Davis should have sought assurances that Corrections was doing all it could to keep the public safe.
Prime Minister Jacinda Ardern said that the alleged offender should not be allowed to spread his message of hate.
But she did not expect Davis to apologise for this "systematic failure".