Defence Minister Hishammuddin Hussein has declined to reveal when military attache Muhammad Rizalman Ismail will be sent back to New Zealand to be tried for sexual assault and burglary.

Speaking to reporters today, he said, the mater is completely under the purview of the Foreign Ministry led by minister Anifah Aman.

"As you know the matter is under the purview of the Foreign Ministry and Anifah Aman (the minister) is working very closely with me," Hishammuddin said when asked whether Rizalman would sent to New Zealand tomorrow.

This follows a report by the New Zealand Herald that Rizalman ( left ), who was attached to the Wellington High Consulate, may not be fit to fly tomorrow and may require more "psychiatric evaluation".

Hishammuddin also brushes off claims that the Malaysian government had attempted to cover up the matter by seeking to invoke diplomatic immunity for Rizalman and for documents on the matter be sealed.

He was responding directly to Opposition Leader Anwar Ibrahim, who made the accusation in New Zealand media.

"For him, everything is a cover up," Hishammuddin said. 

The Umno vice president also cautioned against a "media or public opinion trial" against Rizalman, who is charged of committing the offences against a 21-year-old woman in Wellington on May 9.

"Justice is not only for the accuser, but also for the accused," Hishamuddin told reporters during a press conference after launching a MH370 exhibition today.

"What is wrong is wrong. That is why justice must not only be done, but must also be seen to be done. That is why we made the decision to send him back to New Zealand," Hishammuddin explained.

"The decision was taken at the cabinet level. And the temperature of this case has also dropped significantly since when the news first broke out," he added.