A legal expert suspects the refusal to waive diplomatic immunity for a US embassy official has come directly from Washington.
Police want to speak to Colin White following an incident at an address in Lower Hutt last Sunday, but due to his status, they aren't allowed.
Waikato University's Professor Al Gillespie says usually allies waive immunity in these circumstances, but the US seems to have enforced a blanket rule not to.
"I suspect what's happened here is you're facing a policy which has been directed from Washington, whereby no matter which country they're in, they're not going to waive immunity."
Mr White left the address, in the suburb of Tirohanga, with a broken nose and a black eye, according to reports.
"We take seriously any suggestion that our staff have fallen short of the high standards of conduct expected of US Government personnel," the US embassy said in a statement.
"Any allegations of wrongdoing are always fully investigated. We are communicating with New Zealand authorities."
Prof Gillespie says international law seems unfair in these circumstances, but they've been this way for more than 3000 years and won't ever change.
In 2016 Muhammad Rizalman bin Ismail, who worked at the Malaysian High Commission, was sentenced to nine months' home detention after assaulting a Wellington woman.
The case caused controversy after it was revealed he was allowed to return to Malaysia following a blunder by the Ministry of Foreign Affairs and Trade.