Norway violated mass killer Anders Behring Breivik's human rights by keeping him in isolation in prison after being sentenced for killing 77 people in twin attacks in 2011, a Norwegian court has ruled.

Breivik took Norwegian authorities to court in March, accusing them of exposing him to inhuman, degrading treatment or punishment in breach of the European Convention on Human Rights.

He protested his isolation from other inmates and from outsiders who are not professionals.

"The prohibition of inhuman and degrading treatment represents a fundamental value in a democratic society. This applies no matter what - also in the treatment of terrorists and killers," judge Helen Andenaes Sekulic said in her ruling.

The verdict said the Norwegian state had broken Article 3 of the convention, pointing to the fact that Breivik is spending 22 to 23 hours a day alone in his cell.

"It's a completely locked world with very little human contact," it said, adding that there had been no attempt to ease the security "even though Breivik has behaved in an exemplary manner during his time in prison".

His isolation is "an inhuman treatment" of him in the meaning of the European convention, it said, noting that all his visits, except for his mother who died in 2013, are from professionals and only held through a glass wall.

The wall must be seen as a "completely exaggerated security measure," said the verdict.

The ruling, however, said the Norwegian state had not violated Breivik's right to a private and family life.

Breivik killed eight people with a bomb in Oslo and then gunned down 69 people, many of them teenage members of the then ruling Labour Party, at a gathering on an island nearby.

He is serving Norway's maximum sentence of 21 years, with the possibility of extensions.

In March, the case raised dismay, and some laughter, among Norwegians taken aback by Breivik's complaints of cold coffee and microwaved meals he said were "worse than waterboarding".

Breivik's lawyer said prison authorities must ease the isolation of his client.

"He must first and foremost be allowed to be in contact with other people," Oeystein Storrvik told reporters after the verdict.

He declined to say what Breivik's reaction was to the ruling.

Lawyers representing the state said they would they would reflect would consider whether to appeal.

"We are surprised by the verdict," said Marius Emberland, one of the two lawyers representing the state.

One survivor of the shooting on Utoeya island said the verdict was a sign that Norway has a working court system, respecting human rights even under extreme conditions.

"It also means we have to take the ruling seriously and evaluate how we treat prisoners, what abuses they may suffer, and how we avoid abuse," survivor Bjoern Ihler said on Twitter.

The state must pay Breivik's legal fees, the judge ruled.

Ahead of the verdict, lawyers for both parties said they would appeal if it did not go in their favour.

Breivik's lawyer said his client would not appeal the part of the verdict that ruled against his client.