Director of the Security Intelligence Service Rebecca Kitteridge briefed MPs on the number of Kiwis being spied on since the March 15 terror attack.

Spy agencies are investigating more Kiwis for posing a terror threat, some inspired by the March 15 terror attack.

Between 30 and 50 people are being actively investigated at any given moment by New Zealand's spooks, a higher number than in recent years.

Security Intelligence Service (SIS) director Rebecca Kitteridge​, fronting a parliamentary select committee of high-ranking MPs on Wednesday, said many of those being investigated held racist, Nazi, or white supremacist beliefs.

STUFF Government Communications Security Bureau director Andrew Hampton said the security agenices had been monitoring people for threats to public safety. (file photo)

"[The terror attack] has given encouragement to some people, it has been inspirational to other people, so it remains quite a fluid picture."

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National MP Amy Adams questioned Kitteridge on why the terror threat level remained "medium" — "a terrorist attack is assessed as feasible and could well occur" — nearly a year after the March 15 attack.

JONATHAN ERNST/REUTERS Spy agencies received 455 "leads" on people of concern in the three months after the March 15 terror attack.

"One of the terrible outcomes of the 15th of March attacks, it shows New Zealand that it's a possibility, and we have yet to see how it is going to play out domestically," Kitteridge said.

The spy chief said there had been a high number of "leads" received about people of concern, including 455 pieces of information in the three months following the attack.

She acknowledged this meant the SIS had now investigated people who were not previously considered a possible risk before the attack.

STACEY KIRK/STUFF Spy chiefs Rebecca Kitteridge and Andrew Hampton at an earlier hearing with MPs. (file photo)

"Some of those people existed beforehand, and then there is the impact of the attacks afterward," she said.

Previous years have seen the agencies maintain a list of up to 40 people concerned as concern.

Kitteridge, after a second hearing with MPs that was closed to the public, said those people being investigated did not belong to a single group and could be difficult to monitor.

"They tend to live online and they tend to live in encrypted chat rooms. So very, very challenging for law enforcement and intelligence agencies," she said.

She would not comment on whether "specific attack planning" had been identified in the past year.

"We don't see anything that should cause people to worry about just going about their daily lives. If they were ever to see specific attack planning, we would be working with police immediately to disrupt that."

Government Communications and Security Bureau (GCSB) director Andrew Hampton, also at the hearings on Wednesday, said the job of the spy agencies was to assess threats to public safety.

"It's not about do these people hold views which may be unsavoury, it's about are they mobilising to violence."

Both spy agencies continued to monitor terror threats from believers of Islamic extremism, such as those following the views of Al Qaeda.

Kitteridge said she was not aware of any Kiwis who had returned to New Zealand after joining the so-called Islamic State.