The Prime Minister doesn't want to name which "two or three" countries are responsible for a number of cyber attacks on New Zealand.
The comment comes a day after the acting head of the GCSB, Una Jagose, gave a speech about the increase in attacks on government departments, nationally significant businesses and organisations and research institutes.
Ms Jagose said the spy agency's new cyber security programme, Cortex, has been successfully used to protect its clients, including officials in a "key" government agency.
Since it started in March, Cortex has resolved more cyber security incidents in the first 10 weeks than in all of 2014.
In the 12 months to December 31 last year, there had been 147 hacking incidents reported, with 132 reported in the first six months of this year.
Ms Jagose expected that figure to double by the end of the year.
John Key says the Government is increasing its defensive efforts against foreign cyber attacks, which are increasingly targeted at the private sector.
During a media stand-up in New York today, Mr Key said there were "two or three" countries they knew of which could be behind a larger proportion of the attacks, but he wouldn't say whom they were.
"We don't want to get into naming the countries. We can, behind the scenes, identify two or three countries that are more active than others, but actually it's just good practice for our companies and for central government to protect itself from cyber risk wherever it is coming from," he says.
When asked specifically why he wouldn't name China as the origin of some attacks, Mr Key said there wasn't much point in naming names.
"We could do that, but it doesn't take us far."
In May last year, China was considered the main suspect in hacking a NIWA supercomputer because the attack originated from there.
Ms Jagose's speech, hosted by the Privacy Commission, had to be postponed from earlier this month after it was disrupted by two protesters.