New Zealand's national censor says it would be powerless to block the planned livestream of a US military execution, even as rights groups urge governments to protest the broadcast.

The Classification Office said it "cannot classify a livestream event as it is taking place" and that no body in New Zealand has the authority to block access to a livestream. Chief Censor Caroline Flora said the legal threshold for banning content is high and must balance freedom of expression against the likelihood of harm.

The row follows the Pentagon's announcement that the December 3 execution of Nidal Hasan by firing squad would be livestreamed. Hasan, a former Army psychiatrist, was convicted of killing 13 people at Fort Hood in Texas in 2009. Defence Secretary Pete Hegseth called the method a "no-brainer".

The news carries painful resonance in New Zealand, where the 2019 Christchurch mosque attacks that killed 51 people were livestreamed on Facebook. The country has since led global efforts against violent online content.

For now, the censor's message is blunt: however distressing the footage may be, it is not always unlawful.