Police commando Sirul Azhar Umar, who was sentenced to death for the murder of Altantuya Shaariibuu, should be given a fresh trial, PKR de facto leader Anwar Ibrahim said.

In an interview with The Australian, Anwar said Sirul's original trial judges were "compromised" as the relevant witnesses were not called, thereby "made a mockery of the law".

"The best way is to proffer a new charge and allow for a full hearing of the case," he said.

In a separate statement, Bukit Gelugor MP Ramkarpal Singh said the new Pakatan Harapan government should take the necessary steps to investigate the motive behind Altantuya's murder. 

As a final judgment has also been given in the murder trial, ordering a retrial would contravene the Federal Constitution, thus, investigations into her murder should be reopened immediately to find out who ordered her killing, he noted.

"If it is found from those investigations that certain parties instructed the murder of Altantuya, those parties must be charged for her murder as well for having shared a common intention with her killers, even though they did not pull the trigger," he said, further urging the setting-up of a Royal Commission of Inquiry (RCI) into the matter.

In 2009, Sirul and Azilah Hadri - both members of then deputy prime minister Najib Abdul Razak's security detail - were found guilty of murdering the Mongolian national three years earlier.

Both were acquitted in 2013. Sirul then escaped to Australia and has been held at Sydney's Villawood detention centre since, where he is seeking political asylum.

Their acquittal was eventually overturned by the Federal Court and their death sentences reinstated. 

Defence contractor Abdul Razak Baginda - known to be one of Najib's closest confidante - was initially charged with abetting the duo, but he was eventually acquitted.

The courts heard that Altantuya accompanied Abdul Razak to Paris while the latter was negotiating the deal to acquire Scorpene submarines for Malaysia. Najib was the defence minister at the material time.

Altantuya was killed in 2006, shot twice in the head before her body was disposed of in a forest on the outskirts of Kuala Lumpur with military grade explosives.

The motive for the crime and the source of the policemen’s orders remain unknown.

Following the fall of the BN federal government, there have been calls for Prime Minister Dr Mahathir Mohamad's government to reopen several prominent cases, among them 1MDB and the Altantuya murder.

Former Federal Court judge Gopal Sri Ram has also called for a fresh probe into the Scorpene case and for Abdul Razak to be investigated once more.