North Korea's nuclear test has been condemned by Foreign Minister Gerry Brownlee as "utterly deplorable".
Pyongyang has today claimed it has successfully conducted a test of a hydrogen bomb meant to be loaded into an intercontinental ballistic missile.
"This is the sixth nuclear test North Korea has conducted and appears to be the largest yet," Brownlee said.
"It follows two such tests last year and more than a dozen missile tests this year.
A North Korean TV anchor announced the test's success on Korean Central Television, hours after Seoul and Tokyo detected unusual seismic activity at North Korea's nuclear test site.