Malaysian defence attaché Muhammad Rizalman was covered by diplomatic immunity, despite not having a diplomatic passport.

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In May this year, New Zealand authorities charged him with burglary and assault with intent to rape, but he claimed diplomatic immunity and fled the country.

He appeared in court in New Zealand before being released on bail with certain conditions, but a botch-up by the Ministry of Foreign Affairs and Trade (MFAT) allowed the 35-year-old to return home.

MFAT has confirmed that Mr Rizalman was travelling on his personal passport, but said he was still covered by immunity, and Prime Minister John Key today agreed.

"He is because of his status in New Zealand able to claim diplomatic immunity, and that was the position that the Malaysian authorities in New Zealand invoked, despite the fact I think New Zealand would not want them to do that."

The Malaysian government has now received a formal extradition request for Mr Rizalman.

Mr Key said how long that takes to process will depend on whether Mr Rizalman challenges it in the courts.

Malaysia says the extradition request now has to go through the due process of law and rejects any suggestion it has been dragging its feet.

That is primarily in response to criticisms from opposition parties in Malaysia that delays in extraditing Mr Rizalman could tarnish Malaysia's international reputation.

Ambassador Zulkifi Yaacob is handling the case in Malaysia. He says the Attorney-General's office is preparing the papers that then have to go to the Home Minister for approval.

Mr Yaacob said once that had happened, a date for the court hearing will be set and papers issued for the arrest warrant for Mr Rizalman; they would be served by the Malaysian authorities.

He said the whole process could take up about two to three weeks - but could be longer if Mr Rizalman challenged the warrant once he appeared in court.

In response to questions about why it has taken several months to formally lodge the extradition request with Malaysian authorities, MFAT said that was partly because there was no extradition treaty between New Zealand and Malaysia.

It said lawyers from both governments had been sharing documents and working through technically complex legal arrangements.