A US diplomat has been expelled from New Zealand over allegations of involvement in a “serious” criminal incident.

New Zealand asked the unnamed diplomat to leave the country after the US Embassy refused to waive his diplomatic immunity so that he could be investigated.

Police said officers responded to an incident close to Wellington where the US Embassy is located last Sunday but the American had already left the scene.

Authorities did not release any further details about the incident.

TVNZ identified the diplomat as Colin White, a technical attaché who worked at the US embassy alongside his wife. White reportedly sustained a broken nose and a black eye during the mysterious incident.

Police asked the Ministry of Foreign Affairs and Trade (MFAT) to waive the diplomat’s diplomatic immunity so they could carry out an investigation.

The ministry agreed to the request because the man was involved in a "serious" crime, one that could have seen him land in jail for at least one year.

Subsequently, the ministry appealed to the US Embassy to waive the immunity, but the US government refused to do so.

MFAT officials then asked the American to leave New Zealand immediately.

It is understood that the diplomat has since left the country. However, the case is not closed and the investigation is still “active,” according to police.

Foreign diplomats in New Zealand have immunity from any criminal processes. However, MFAT will request that their immunity be lifted “if there are allegations of serious crimes.”