Sirul Azhar Umar told Malaysia's online news website Malaysiakini that he has been negotiating with Australian television stations for a politically explosive interview where he would reveal who ordered him to shoot Mongolian socialite Altantuya Shaariibuu as she begged for the life of her unborn baby, before blowing her up in a patch of jungle on Kuala Lumpur's outskirts.

"Generally, the department does not facilitate interviews within detention facilities for operational reasons," said a spokeswoman for Immigration minister, Peter Dutton.

Sirul, a 43-year-old former bodyguard of Malaysian Prime Minister Najib Razak, would need top-level approval to record a media interview in Villawood's maximum security Blaxland unit, where he has been held since being arrested in Queensland on January 20.

The spokeswoman would not rule out the possibility of an interview in the future if the proper application processes were met.

In a rare comment on the case, Mr Najib dismissed as "rubbish" a claim by Sirul that he was ordered by "important people" to kill Shaariibuu, who had been a translator in a  $US2 billion deal for Malaysia to buy two French and Spanish-built submarines that is the subject of a corruption investigation by French magistrates.

"It's total rubbish. Total rubbish," Mr Najib said when questioned about Sirul's claim by a reporter in Kuala Lumpur.

Sirul faces indefinite detention on immigration charges because Australia has made clear it will refuse a Malaysian request to extradite Sirul unless the government in Kuala Lumpur gives an undertaking that he would not be executed upon his return.

Allegations have simmered for eight years that Shaariibuu was murdered to keep her quiet about purported kickbacks to high-level Malaysian officials over the purchase of the submarines when Mr Najib was defence minister.

Mr Najib has strongly denied ever meeting Shaariibuu and his government denies any wrongdoing in the deal.

For years the case has been largely taboo in Malaysia's government-controlled mainstream media, with developments generating intense interest in social media and on news sites like Malaysiakini and the Hong Kong-based Asia Sentinel.

Sirul and his former police colleague Azilah Hadri were sentenced to hang on  January 13, after Malaysia's Federal Court upheld a previous murder conviction.

When Azilah was led to death row to await execution Sirul had fled to Queensland where he was staying with relatives.

In his comments to Malaysiakini, Sirul expressed frustration with the legal proceedings and the way he has been treated by his former bosses in Malaysia's police force.

After being allowed to hide his face during months of court hearings, Sirul criticised Kuala Lumpur's police chief Khalid Abu Bakar for releasing his photograph to the media in January. "Why reveal the photograph without my consent? Since the start of the hearing and up to the appeal at the Federal Court, I managed to hide my face," he said.

Controversy over the case has revived as Mr Najib faces intense criticism from conservative figures within his ruling coalition, including abrasive former prime minister Mahathir Mohamad.

The government has been condemned internationally for relentlessly pursuing opposition leader Anwar Ibrahim, who last week was jailed for five years on a rarely used charge of sodomy amid widespread perceptions that his prosecution was motivated by political revenge.

Malaysia also faces sliding oil and gas revenues and allegations of mismanagement in state venture fund 1Malaysia Development Berhad, headed by Mr Najib, that faces losses of billions of dollars.