The heads of New Zealand's spy agencies say they've investigated all kinds of extremist threats in the past, but the Royal Commisson will get to the bottom of whether they could have prevented the Christchurch terror attacks.

Photo: RNZ / Jane Patterson

Rebecca Kitteridge from the SIS and the GCSB's Andrew Hampton went to Parliament today to brief MPs about foreign interference on general elections, where they sounded the alarm about foreign interference in New Zealand's political system.

Afterwards they spoke to reporters about the mass shootings.

A Royal Commission was launched at the start of the week, with a specific focus on the role of the intelligence agencies.

There have been significant questions about whether the spy agencies were paying enough attention to the threat posed by right wing white supremacists in the lead up to the Christchurch terror attack.

The Royal Commission will look at what the agencies did or did not know about the gunman's activities before the attack, what actions, if any, were taken, whether there were any impediments to information sharing with relevant state sector agencies, and whether their counter terrorism priorities were appropriate.

Speaking publicly for the first time since the attack, Ms Kitteridge offered her deep sympathy to the victims and the Muslim community.

Outside the committee hearing, she was asked whether they've been too focused on Islamic extremism.

"We're focused on extremist behaviours of all kinds, so it's ideology neutral; what we do respond to is the presenting threat in society - that was not just extremism prompted by Sunni extremism for example."

She had personally worked closely with the Muslim communities when they had raised concerns about their safety, including from the risk of attacks inspired by ISIS.

Mr Hampton said the GCSB could only help other agencies when they had a firm lead, and the gunman was not on authorities' radar.

"How intelligence gathering works is you need to have a lead or hypothesis, you need to know what question to ask - otherwise you are literally looking for a needle in a very big haystack."

It has always been seen as very difficult to prevent a lone wolf attack, said Ms Kitteridge.

"Having said that, of course, it's important that the questions are asked whether these attacks could have been prevented. And so we will go into the Royal Commission, fully openly offering what we knew, what could we have done, along with the other security sector agencies."

She said she and the intelligence community were horrified by the attack.

"My heart goes out to those families... we've been deeply affected by what happened and working tirelessly since the attacks to to ensure that we understand the intelligence picture, support the police investigations and prevent copycat attacks."