Prime Minister Bill English hasn't seen an explanation as to why the United States refused to waive immunity for one of its diplomats wanted by police.

Speaking at his regular post-Cabinet press conference today, English was asked why the US had not agreed to waive immunity for the embassy staff member, thought to be named Colin White, over an incident in Lower Hutt on Sunday, March 12.

He said he "hadn't seen an explanation for that, other than the decision not to waive".

"We expect all diplomats here to obey our law, and if it is broken we would expect our police to investigate," English said.

"There are a set of procedures around how these things are dealt with with diplomats. We regret they didn't give us a waiver on immunity but they didn't. And now it is in the hands of their authorities.

"I don't think it will affect it [the US-NZ relationship] significantly. There may be some further discussion to be had, but the issue is now with the US authorities."

Asked if the decision not to waive immunity reflected a new policy under the Trump Administration, English said that was a question for US authorities and he was unable to answer it.

It is not known what occurred in the incident, but <i>TVNZ</i> reported that White was left with a broken nose and a black eye.

The Ministry of Foreign Affairs and Trade (Mfat) will request a waiver of immunity from a diplomat's government if requested by police and if the allegations relate to an offence for which the penalty is a term of imprisonment of 12 months or more.

It asked the US Embassy to waive White's diplomatic immunity so police could investigate, and when that request was declined asked the US to withdraw White from New Zealand. He has now left the country.

A US Embassy 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," she said. "Any allegations of wrongdoing are always fully investigated."

Foreign Minister Murray McCully has said he is satisfied with the way Mfat had conducted the process.

"Officials in Wellington and our Ambassador in Washington DC have clearly conveyed to the United States the expectation that foreign diplomats obey the law in New Zealand and are seen to face justice in New Zealand.

"The refusal to waive immunity is disappointing, and as a result MFAT has asked that the diplomat be withdrawn from New Zealand."

Labour foreign affairs spokesman David Parker said MFAT followed the correct process and he hoped that the US investigations would now "make the proper inquiries".

NZ First leader Winston Peters, a former Foreign Minister, said New Zealand now had to wait for the US to investigate.

"These are allegations which we expect the Americans to fully investigate and act upon, and advise us of their actions," he said.