Sirul Azhar Umar has been transferred to Villawood detention centre in Sydney's western suburbs after his arrest at his family's home in Ipswich, Queensland, on January 20.

The 43-year-old former bodyguard of Malaysian Prime Minister, Najib Razak, has been convicted of the murder of a glamorous Mongolian model who begged for the life of her unborn baby before being shot twice in the head, wrapped in C-4 plastic explosives and blown up on the outskirts of Kuala Lumpur in 2006.

Allegations have simmered for eight years that 28-year-old Altantuya Shaariibuu was murdered to keep her quiet about purported kickbacks to high-level Malaysian officials.

Ben Saul, a professor of international law at the University of Sydney said that under Australian law Sirul cannot be extradited due to the death sentence and cannot be let free because of the murder conviction against him, leaving him in legal limbo.

Dan Mori, the former lawyer for David Hicks agreed with Professor Saul. "It is unlawful arbitrary detention," he said.

Mr Mori is representing accused Serbian war criminal Dragan Vasiljkovic who has been in legal limbo and fighting extradition for nine years.

There are 55 cases of indefinite detainment without charge in Australia. They are defined by an "adverse ASIO assessment that puts them at risk to the community" said Professor Saul.

"According to the UN It is absolutely clear that in these cases of indefinite detainment Australia is in breach of International law," said Professor Saul.  "But this is an easier option than dragging a foreign national through the criminal courts."

In January, the federal government came under fire from the president of the Australian Human Rights Commission Gillian Triggs for the indefinite detention of an Indonesian man charged with murder, prompting Prime Minister Tony Abbott to label her judgment "extremely questionable".

The Department of Immigration and the Attorney-General's department have declined to comment on the Sirul case.

The legal and diplomatic stalemate comes as Sirul's mother, Piah Samat, 74, said "utter nonsense" has been spread about her son.

There are some who allege he has been paid off when he didn't receive anything," the Malaysian Harakah Daily quoted her as saying. "I just pray to God he will be safe."

Opposition MPs in Malaysia have called on Sirul to publicly reveal the motive behind the murder of Ms Shaariibuu.

Ms Shaariibuu, described as a sophisticated jet-setting party girl, worked as a translator in the later stages of negotiations over the controversial $2 billion purchase of two French- and Spanish-built Scorpene submarines when Mr Najib was defence minister.

Mr Najib strongly denies ever meeting Ms Shaariibuu or having any link to her. His government denies any wrongdoing in the submarine purchases which are the subject of an investigation by magistrates in France.

Sirul's arrest has created a diplomatic dilemma for the federal government as it is expected to strongly argue that Malaysia should not hang Sydney grandmother Maria Elvira Pinto Exposto if she is convicted after being arrested in Kuala Lumpur allegedly in the possession of illegal drugs.

Malaysia has said it will take legal action in Australia if the government does not extradite Sirul under an extradition treaty between the two countries.

The only course of appeal remaining for Sirul is for a Malaysian sultan to commute the death sentence to life imprisonment, which could reopen the possibility of extradition.