Brenton Tarrant is the first person in New Zealand history to be charged under section 6A of the Terrorism Suppression Act 2002.

And a constitutional law expert says, if convicted, the accused gunman could spend the rest of his life behind bars.

However, the new terrorism charge will not make a "great deal of difference" to the alleged gunman's overall sentence, Wellington barrister Graeme Edgeler told the Herald.

"The murder charge is the most serious one, for the murder charge you can get life without parole.

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"Given the seriousness of the events, the number of charges, the number of people killed and things like that, life without parole could be a possibility.

"For the terrorism suppression charge, the charge of committing a terrorist act, he could get a life sentence with a non-parole period of 10 years."

If convicted of murder the alleged gunman could be dealt life in prison without parole, a first in this country.

Edgeler believed the terrorism suppression charge was handed down to recognise the wider impact felt following the terrorist attacks.

Using a different section of the Terrorism Suppression Act, police looked at prosecuting under the act in the Urewera raids case in 2007 but decided not to.

Police Commissioner Mike Bush. Photo / Mark Mitchell
Police Commissioner Mike Bush. Photo / Mark Mitchell

Earlier today, Police Commissioner Mike Bush announced the charge had been filed under the Act.

"The charge ... follows consultation between Police, Crown Law and the Christchurch Crown Solicitors Office," he said.

"An additional murder charge and two additional attempted murder charges have also been filed."

In total, the alleged gunman now faces 51 charges of murder and 40 of attempted murder following the attacks on two Christchurch mosques on March 15.

A judge could not hand down a non-parole period of more than 10 years under the terrorism suppression charge, Edgeler said.

"[The gunman] could be kept in forever on it but he would be eligible for parole on that offence after 10 years," he said.

"If he's convicted of something he'll be convicted of murder and that'll be a non-parole period which will determine if he is ever eligible for parole."

The Solicitor General is required to sign off on any charges laid under the Act, Edgeler said.