New Zealand Foreign Affairs Minister Murray McCully today apologised for a misunderstanding that led to a Malaysian military attache claiming diplomatic immunity and leaving the country after being arrested on sexual assault charges.

Muhammad Rizalman Ismail, 39, was arrested in May 2014 after following a woman home and entering her bedroom naked from the waist down. He claimed diplomatic immunity and left the country, but was later returned to face the charges.

McCully said an email from New Zealand's Foreign Affairs Ministry had mistakenly given Malaysian officials the impression that it would be acceptable for Rizalman to return home after his arrest.

"This should never have been in any doubt. I would like to acknowledge the role of the Malaysian government in ensuring that he returned to face justice in New Zealand.

"I would also like to reiterate my apology to the young woman who was so badly let down by the way the issue was handled," McCully said.

The Foreign Affairs Ministry has introduced clearer policies on when officials should seek a waiver of diplomatic immunity as a result of the case.

"As a matter of explicit policy, we will seek a waiver of diplomatic immunity for all serious crimes where the police determine that a prosecution is warranted," chief executive Brook Barrington said.

A report into the case found the events could have put New Zealand's relationship with Malaysia in jeopardy, had it not been for mutual efforts to build a strong and positive relationship over a number of years.

Rizalman was yesterday sentenced to nine months home detention, to be served in New Zealand, and ordered to pay reparations to his victim Tania Billingsley, who was 21 at the time of the assault. She voluntarily had her name suppression lifted to speak out on the case.

Rizalman will be deported after serving his sentence.

- dpa