The minister in charge of New Zealand's spy agencies, Andrew Little, says he has signed off more surveillance warrants since the Christchurch terror attack on March 15.

But, speaking to the Herald, Little would not say exactly how many people were now being spied on as a result of the attack.

"There is usually between 30 and 40 people at any one time at various states of being either under surveillance or considered for it.

"That number will be bigger now and it will be at various ends of the people-of-interest scale."

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Little said he had "authorised new activity", such as surveillance and intrusive activity, since the attack.

But he wouldn't detail numbers or the nature of that activity.

He said a warrant could be anything from physical surveillance to telecommunications activity.

"The whole gambit of what would otherwise be described as intrusive activity.

"The purpose of a warrant is to authorise and effectively make lawful what would otherwise be unlawful activity."

Little is the minister responsible for the Security and Intelligence Service (SIS) and the Government Communications Security Bureau (GCSB).

He has the final say over which people the agencies spy on and signs off all warrants.

Speaking to the Security and Intelligence Select Committee last month, NZSIS Director Rebecca Kitteridge said in terms of terrorism and violent extremism, at any one time around 30 people were of particular interest to the SIS, although the number was not static.

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As a result of the investigations into the Christchurch attacks, Little said: "new activity is leading to other individuals being identified".

"All individuals identified need to be the subject of some level of investigation; some of those may find that they are under a more intense level of investigation, including surveillance."

He was not able, however, to go into much detail without compromising the ability for those investigations to be conducted.

Little wouldn't say if the people, who had come under surveillance since the attacks, were white extremists.

"The agencies have responsibilities for investigating terrorism and violent extremism of all sorts.

"That is the category of work that is leading to more people being either considered persons of interest and being put under varying levels of investigation and surveillance."