Answer not easy, but increasing attacks, security and controls only brings more terror, professor says.

Kiwi political leaders and terror experts agree attacks like those in Brussels are difficult to prevent.

Prime Minister John Key yesterday said New Zealanders would be appalled by the "barbaric" attacks in the Belgian capital.

"Our hearts go out to the people of Brussels but also to the people of Europe who have really now had a second major attack in not a long period of time, following on from the Paris attacks."

Mr Key stressed the importance of intelligence gathering by New Zealand spy agencies and partner countries to help stop terror.

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"These attacks are very difficult to stop in a practical sense, when someone just walks in with suitcases and detonates a suicide bomb," he said.

"We really have to stop these acts before they are actually carried out."

University of Otago terrorism expert Richard Jackson said attacks in Europe and the rest of the world would continue until world leaders changed the way they responded.

He said they often reacted to terror by increasing security, intensifying military attacks on regions terrorism was thought to be coming from, increasing restrictions on civil liberties, increasing surveillance and targeting of Muslim communities, and introducing "draconian" legislation.

"And the only thing that has achieved is more terrorism," he said.

"It's impossible to secure everywhere in modern society from these kinds of attacks. So to say we are going to make our society secure by increased security measures and by vigorous military action is either a lie or it's a delusion."

When asked what the alternative to that would look like, he said: "The first thing to do would be to undertake an honest and rigorous investigation into the root causes of this violence - to try and understand where it comes from."

International security analyst Paul Buchanan said the attacks were likely to lead to a clampdown on security at public transport hubs, including invasive front door inspections at all airports and train stations.

It was also likely to threaten freedom of movement in and around the European Union, with a possible end to the Schengen Agreement that allowed open borders between nations.

New Zealand's terror threat level remained unchanged yesterday.

Domestic spy agencies are closely watching between 30 and 40 individuals who are considered a security risk. That number has not changed in 18 months.