New rules for inmates' mail are expected to take into account the danger of social media

Changes to the Corrections Amendment Bill are designed to counter radicalisation or discrimination in prisoners' mail - and the bill is expected to pass its third reading tomorrow.

The changes follow a review into whether the law was fit for purpose after Corrections failed to vet the mail sent from the accused Christchurch gunman.

New Zealand Howard League for Penal Reform CEO Mike Williams says its unfortunate the current system has been exploited.

He says the rules were in place before social media rose to such prominence, and there's probably a lack of training for Corrections staff.