Police told the Herald on Sunday they had sufficient evidence to charge him with assault with intent to rape, but had let him go as he was not able to be prosecuted under New Zealand law. He had also been charged with burglary.
It is understood the man — who was aged in his 30s and had interim name suppression — refused to give a DNA sample.
He had full diplomatic immunity, and left New Zealand.
Police spokesman Nick Bohm said the complainant "has been fully informed throughout the process and we are continuing to support her".
A spokesman for the Ministry of Foreign Affairs and Trade (Mfat) said the diplomat was immediately sent home after his government refused to waive his immunity.
"The New Zealand Government expects diplomats to respect New Zealand laws. However, under the Vienna Convention there is global recognition that diplomats are entitled to immunity from arrest and detention," the spokesman said.
Under the convention, diplomats, their immediate family and staff cannot be arrested or detained. Their houses cannot be searched and nor can they be called as witnesses in a prosecution or be prosecuted.
It is understood the Government expects the man will be investigated by the authorities in his country, and is monitoring the situation.
Mfat said serious crimes by envoys in New Zealand were rare, but Louise Nicholas, survivor advocate for Rape Prevention Education, has labelled the diplomatic immunity a "crime in itself".
"It's really difficult for the Government. There is nothing you can do about it. The victim has to live with knowing there will be no justice for her. It is disgusting of the [diplomat's] government to allow this man home.
"For these people to be able to legally walk free is a crime in itself."
Nicholas praised the NZ Government's attempts to get the man's immunity waived, and still wants him to be extradited.
"The crime was committed here so he should be brought back here to face the judicial process ... It's just so wrong."
