Govt powerless on diplomat sex attack: Key

There's nothing the New Zealand government can do to prosecute a man who used diplomatic immunity to avoid facing a charge he tried to rape a Wellington woman.

Police on Sunday confirmed they arrested a man in his 30s in Brooklyn on May 9 and charged him with burglary and assault with intent to rape, after he followed a 21-year-old woman home.

However, the man - whose identity has been suppressed - has since left New Zealand after his home country declined a request from the Ministry of Foreign Affairs to waive diplomatic immunity.

Under the Vienna Convention, diplomats are entitled to immunity from arrest and detention.

Prime Minister John Key said the matter is now in the hands of the home country.

"My understanding is that the home country is looking at the case and about whether charges might follow there," he told reporters.

Mr Key would not confirm whether or not his government had formally expressed its displeasure over the scandal, but said local authorities would have prosecuted the man if he did not have diplomatic immunity.

"We certainly expressed our view that there was an argument, as I understand it, that the person should have been charged and faced charges in New Zealand," he said.

"But it is, as you know, up to the home country to decide whether they invoke diplomatic immunity or not."

Police said the charges remain active and the court had issued a warrant for his arrest.

"The victim has been fully informed of the process throughout and we continue to support her."

Labour's foreign affairs spokesperson David Shearer says New Zealanders deserve justice even in cases of diplomatic immunity.

"New Zealanders need to know that this matter will not simply be swept under the carpet and forgotten about it," he said.

"We expect justice to be done and the rights of any victim to be respected and not simply forgotten."

Mr Shearer said the government should demand the home country investigate the charges properly.

"The diplomat's extradition back to New Zealand to face charges should be pushed as an option."