The US Embassy is refusing to allow New Zealand police to speak to an American diplomat involved in an incident in Lower Hutt.

Police want to interview the embassy staff member, thought to be named Colin White, over an incident on Sunday.

It is not known what occurred, but TVNZ reported that White was left with a broken nose and a black eye.

A Ministry of Foreign Affairs and Trade (MFAT) spokesman said it asked the US Embassy on Monday to waive White's diplomatic immunity so police could investigate.

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"The United States Government has today declined to waive the diplomat's immunity," the spokesman said.

"Therefore, MFAT has asked the United States to withdraw the staff member in question from New Zealand."

The US Embassy confirmed it was speaking to New Zealand authorities about the case.

A spokeswoman said the embassy's policy was not to comment on matters which were under investigation.

"We take seriously any suggestion that our staff have fallen short of the high standard of conduct expected of US Government personnel.

"Any allegations of wrongdoing are always fully investigated."