A US Embassy source confirmed on Saturday attache Colin White has left the country.
The man was protected by diplomatic immunity, which the New Zealand police asked the Ministry of Foreign Affairs and Trade (MFAT) to waive on 13 March so they could proceed with an investigation.
1News revealed last night that police want to question technical Chris White over an incident in Lower Hutt on Sunday.
They said they were working with the Ministry of Foreign Affairs and Trade, which asked the United States to waive immunity on Monday.
TVNZ reported the USA diplomat at the centre of the incident was Colin White, a technical attache who works at the U.S. embassy in Wellington alongside his wife.
Foreign Minister Murray McCully says it's "disappointing" that the United States government has refused to waive immunity for one of its diplomats.
A serious crime was considered to be an offence for which the penalty was a term of imprisonment of one year or more.
The matter remains under active investigation, and police have also been liaising with MFAT as part of ongoing enquiries.
The man - who has not been named - was asked to leave the country after the USA embassy refused to waive his diplomatic immunity so that he could be questioned by police.
Mr McCully says he is satisfied with the way the ministry has conducted the process.
Diplomatic immunity, formalised in the 1961 Vienna Convention, means foreign envoys are protected from local law enforcement in the country to which they are posted.
The embassy told TVNZ: 'We take seriously any suggestion that our staff have fallen short of the high standards of conduct expected of US Government personnel'.
"Any allegations of wrongdoing are always fully investigated".
He left New Zealand shortly after the incident, but had to return to New Zealand to face trial.
The refusal to waive immunity is disappointing and as a result MFAT has asked that the diplomat be withdrawn from New Zealand.
A USA government spokesman says as a matter of policy "we do not comment on the specifics of matters under investigation".