A top criminal defence lawyer says it won't be an easy call committing the Christchurch Mosque shooter to life behind bars - despite the magnitude of his crimes.

The landmark four-day sentencing will get underway tomorrow - after the 29-year-old earlier pleaded guilty to 51 murder charges, 40 attempted murder charges and one terrorism charge for the March 15 attacks last year.

Lawyer Marie Dyhrberg told the Weekend Collective it would still be a major step imposing life without parole on somebody of his age.

"The actual risk for this person throughout the rest of his sentence is going to always be very high, but to actually take away the prospect of ever getting out, it would be a big call."

She says the judge may well consider those guilty pleas when deciding if he has any remorse for what he's done.

"A sentencing judge will take into account, did they plea, not just for convenience, but does it signify this is someone who is capable of rehabilitation in the long run."

But Dyhrberg says the judge may well decide the evidence against him was so overwhelming that all the pleas did was spare the cost of a trial.

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