Labour leader Andrew Little says he is more assured about the activities of the GCSB than he was a week ago, but he said Prime Minister John Key and the minister responsible for the spy agencies, Chris Finlayson, had a duty to explain to the public what was and wasn't happening.
Mr Little was critical last week of suggestions that the GCSB, the Government Communications Security Bureau, was undertaking mass collection of communications in the Pacific to pass on to the United States' National Security Agency, claims based on documents taken by Edward Snowden from the NSA.
"From the public session I take a greater level of assurance than I had perhaps a week ago," Mr Little told the Herald.
But he said questions remained "and I still maintain that it is for political masters of those agencies to be accountable to the public about what is and isn't happening".
He was speaking after the acting head of the GCSB, Una Jagose, and the director of the SIS, Rebecca Kitteridge, appeared before the Intelligence and Security Committee on which Mr Little sits.
After the public sessions, the directors and MPs on the committee headed for a secure room in the Beehive where classified material could be discussed.
In the public session Ms Jagose denied that the GCSB engaged in any indiscriminate collection of communications and said all collection was authorised.
"The very collection of information is authorised ... It is not that we collect information and then seek authorisation for particular targets' issues. Everything we collect is authorised," she said.
She said the terms "mass surveillance" and "mass collection" which had been used a lot were not in the GCSB legislation.
"The connotation I get from those phrases is some indiscriminate, for no purpose, not necessary collection of information for collection's sake and we do not do that.
"What we do is lawful and authorised and necessary and proportionate and all of it ... is subject to independent oversight."
She said the answer to the "real tension" between the bureau's need for secrecy and the public's wish for greater transparency was to have trust in the oversight of the agencies.
The independent Inspector-General of Intelligence and Security, Cheryl Gwyn, was entitled to "and does, come into the bureau at any time and she can look at anything she likes", said Ms Jagose.
When Mr Little asked Ms Jagose what the term "full-take" collection meant, a term used in Snowden's documents on the GCSB, she said he would be better to ask that in closed committee. He said later he could not discuss what was discussed in the closed session.