A unprecedented public talk by the head of the Government Communications Security Bureau (GCSB) has been cancelled after being crashed by protesters.

The speech by acting GCSB director Una Jagose would have revealed new details about the agency's activities, including the secretive Cortex cyber-security programme.

Ms Jagose had been welcomed to the lectern by Privacy Commissioner John Edwards when two protesters stood up and unfurled a banner labelling the talk a propaganda exercise.

When they refused to sit down or leave, despite the pleas of audience members, Mr Edwards made the decision to cancel the highly anticipated event.

The talk was held in an open area in the corner of the National Library of New Zealand's ground floor, in Wellington. Members of the public were asked to preregister to gauge numbers, but there was no visible security.

Afterwards, Ms Jagose said the interruption was very disappointing.

"Protests are a legitimate part of the democracy that we live in, and I like the democracy that we live in.

"So is the work that the bureau does - a legitimate part of the democracy that we live in, so it is a shame that one stopped the other from going ahead today."

She would have spoken about the work the GCSB does in foreign intelligence and cyber defence, and the systems of oversight that are in place, Ms Jagose said.

"I was going to talk a lot more than we have ever talked about before in public about our cybersecurity programme, called the Cortex programme - how we look at the privacy interests related to that programme, how it works, how it is controlled, what it is. Stuff that we have never said before.

"We will look for an opportunity to get that out. Because I think the public want to know. We need to take a breath and find a way to do that, because it is a story that needs to be heard in its entirety."

Cortex is a cyber-shield that is designed to protect Government agencies and other organisations, such as power companies, from cyber attacks.

Its existence was first revealed by Prime Minister John Key before last year's election, and ahead of Kim Dotcom's "moment of truth" event in Auckland.

"What I was going to show is what that is, why it is entirely proportionate to the threats that we see in the cyber world, and why what we are doing is about protecting New Zealanders and New Zealand's interests," Ms Jagose said.

The GCSB is responsible for keeping sensitive Government data secure and also provides protection to selected private companies through a cybersecurity programme.

It has reported advanced attacks as becoming more common.

Asked if serious incidents were becoming more common, Ms Jagose said statistics would be released shortly.

"The threat about cyber is the connectedness to the internet, so as that increases, as it does almost every day, yes the threat increases."