The Commissioner of Security Warrants says very few warrants sought by intelligence agencies are rejected outright by him, but some do have to be tweaked to make sure they comply with the law.
The commissioner, Bruce Robertson, along with Prime Minister John Key, has to sign off on interception warrants that involve New Zealand residents or citizens.
Mr Robertson appeared before Parliament's Foreign Affairs, Defence and Trade Committee, which is considering the new law for the Government Communications Security Bureau (GCSB) and the Security Intelligence Service (SIS).
The legislation sets up the same warranting and compliance regime for both agencies, and allows the GSCB to intercept New Zealanders' private communications.
Mr Robertson, a retired Court of Appeal judge, told MPs the agencies were well-versed in the law, and outlined what kinds of warrants would get his sign-off.
"We are dealing with professional people who can read the [relevant] act as well as I can and know that the old man is not easily fooled, if there is not an evidential base to what is there."
On some occasions he had questioned whether the scope of a warrant was too broad, for example its duration, he said.
"All the time you are there to see the narrowest effective way in which the intrusion can take place.
"And so it would be simply be challenging someone that came up with a proposal saying 'We need to this, this and this', and to say 'Well if you weren't doing that - what disadvantage would there be?'"
Mr Robertson also signed off on warrants under the legislation covering telecommunication companies.
He said one such warrant had been issued, but as that was probably an ongoing matter, he declined to give any further details.