Minister responsible for SIS and the GCSB Chris Finlayson would not comment on what prompted the SIS to carry out surveillance without a warrant.

New Zealand's internal spy agency has carried out warrantless surveillance, utilising new powers granted to it under recent anti-terror legislation.

Such an authorisation can only be granted if there is reason to believe a person or group is planning or about to commit a terrorist act.

The details have been revealed in a report to minister responsible for the GCSB and SIS Chris Finlayson.

Director of Security Rebecca Kitteridge reported that one authorisation was granted between July 1 and December 31 last year.

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"This authorisation was to exercise powers that would otherwise be required to be exercised under an intelligence warrant."

It remained in force for two "12 hour bands", and a warrant was sought retrospectively.

It's not clear whether warrant was granted, but the report from Kitteridge said there had been no determination from either the Minister or the Minister and the Commissioner for Security Warrants, that the authorisation was inappropriate.

Finlayson would not disclose the circumstances under which the authorisation to spy without a warrant was given, but he said it was "very rare".

According to the law, the Director of Security can issue an authorisation to spy without a warrant if they are satisfied it is necessary "for the detection, investigation, or prevention of any actual, potential, or suspected terrorist act; or facilitation of a terrorist act".

"Basically, you would apply for them if there was a particular set of events that necessitated such an order. It would be a reasonably rare occurrence," Finlayson said.

The powers came into effect in 2014, under new legislation to beef up New Zealand's spying powers in response to an increase in global terrorism.

The legislation eventually received cross-party support, but not before Labour managed to claw back a proposed 48-hour warrantless surveillance period, to 24 hours.

The new laws also require agencies to report details of all warrantless surveillance to the minister responsible within six month reporting periods.