Corrections already have the power to make sure the alleged Christchurch gunman doesn't spread his ideologies through his mail, one lawyer says. 

Brenton Tarrant has sent seven letters from prison including two to his mother.

One of them has been posted online and the content has prompted Corrections Minister Kelvin Davis to admit there was a failing in his department.

He says he doesn't think the laws are fit for purpose and he's asked for advice on whether a law change is necessary.

Nigel Hampton QC told Heather du Plessis-Allan no law change is needed.

"All that power is there available to them. It's simply a matter of not having system in place or not sufficiently educating their officers to prevent happening what happened in this letter that went to Russia."

He says that Tarrant should be able to send and receive letters, but they have to be monitored.

"It's quite simply a stuff up. They've got the resources, they've got the ability, they didn't use the resources, they didn't sufficiently use the ability." 

• HDPA: Govt can't blame the law for alleged gunman's letter getting out