Nato boss and former Norwegian Prime Minister Jens Stoltenberg is among the many high-profile names to meet with the Royal Commission investigating the March 15 terror attacks in Christchurch.

Stoltenberg was Norway's Prime Minister when 77 people, mostly teenagers, were killed at a summer camp in Oslo in 2011 by a white supremacist.

He was in New Zealand earlier this month, meeting with Prime Minister Jacinda Ardern and other officials.

At the time, he praised both Ardern and New Zealand's response to the shootings which claimed the lives of 51 people and injured many more.

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"I'm impressed by the way New Zealand handled the terrorist attack in March and the way you stood together; the way you have stood up for ... tolerance, democracy [and] individual liberty."

Submissions to the Royal Commission are done behind closed doors.

But a spokesperson for the inquiry said the commission capitalised on the fact Stoltenberg was in the country so he could speak to the committee.

Stoltenberg is among many people who have personally submitted to the inquiry so far.

Sir William Young, who is leading the Commission, said this morning he had extended the public submission period to give more people a chance to present.

This is the second time the submission period has been extended.

"The information, evidence and submissions we are receiving, are very helpful as we identify and follow a number of leads," he said.

The submission period has been extended to the end of September to enable "valued submissions to be completed".

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How the accused Christchurch gunman acquired guns and a lack of official response to complaints about racial attacks are among the main issues that have been raise during the submission period so far.

The role of social and mainstream media creating division has also been discussed.

Among those who have personally submitted to the commission is Auckland Mayor Phil Goff, former Race Relations Commissioner Susan Devoy and State Service Commission boss Peter Hughes.

Representatives from the Islamic Association, the New Zealand Police and the Institute for Law have also presented to the inquiry, as well as dozens of public agencies including Ethnic Affairs, the Defence Force and New Zealand's intelligence agencies.

Submissions opened July 1 and will now close on September 27.