New Zealand's spy agencies are encountering an increasing amount of "dark communications" that they can't monitor, Prime Minister John Key says.
The weekend killings in Paris have raised questions around interception capabilities and Mr Key says Islamic State has become better at knowing what forms of communication can and can't be monitored.
"The amount of 'dark communications' that can't be monitored by our agencies is increasing," he told reporters in Vietnam, where he is leading a trade mission.
"The level of encryption is very high... my agencies talk to me all the time about whether someone is communicating in a particular way and if so, what they can cover and what they can't."
Mr Key says that going from media reports of the Paris attacks, it seemed there were no communications between the perpetrators.
"So it seems they were obviously aware of the risk of being monitored... the issue is that this technology is very difficult to break into."
The Security Intelligence Service has about 40 people on its watch list and Mr Key says it's possible some of them could be using communications that can't be monitored.
"It's possible, in the spectrum of communications there are some that are far more secure than others," he said.
"People in their daily lives are doing things that are a lot more sophisticated than they were."