New Zealand security services did not focus enough on the threat from right-wing terror in the run-up to last year's Christchurch mosques attack, but could only have prevented the murders "by chance", an inquiry reported Tuesday.

The royal commission -- the most powerful judicial probe available under New Zealand law -- called for sweeping changes to counter-terrorism operations in response to the white supremacist attack in which 51 Muslim worshippers died.

A New Zealand royal commission report on the Christchurch mosques attack found that it could only have been prevented 'by chance'
A New Zealand royal commission report on the Christchurch mosques attack found that it could only have been prevented 'by chance' AFP / Neil SANDS

The 800-page report said spy agencies had placed an "inappropriate" focus on Islamist extremism before the attack and did not sufficiently explore the right-wing terror threat.

But it stopped short of saying authorities could have prevented the massacre, finding that the "fragmentary" information available about lone-wolf shooter Brenton Tarrant prior to the killings was not enough to mark him as a threat.

"There was no plausible way he could have been detected except by chance," said the report on the March 2019 attack, in which Tarrant targeted men, women and children who had gathered for Friday prayers in the South Island city.

New Zealand Prime Minister Jacinda Ardern apologised for the failings identified by the royal commission
New Zealand Prime Minister Jacinda Ardern apologised for the failings identified by the royal commission AFP / Neil SANDS

New Zealand Prime Minister Jacinda Ardern welcomed the review and vowed to look at all 44 recommendations.

She said another major criticism in the report -- lax firearms law enforcement that allowed Tarrant to amass an arsenal of military-style semi-automatic weapons -- had already been addressed by her government.

"The commission made no findings that these issues would have stopped the attack, but these were both failings nonetheless and for that I apologise," she said.

Tarrant, 30, became the first person in New Zealand to be jailed for life without the prospect of parole in August after pleading guilty to terrorism, 51 counts of murder and 40 of attempted murder.

The commission into New Zealand's worst modern-day massacre was originally due to report in December last year but was delayed first by the sheer volume of responses and then by the Covid-19 pandemic.

Headed by Supreme Court judge William Young and former diplomat Jacqui Caine, it interviewed more than 400 people and received 1,100 submissions.