Phuong Ngo was jailed for life without the possibility of parole or review in November 2001 for ordering the assassination of Mr Newman, his political rival in the Labor Party, in September 1994.

Legal Aid lawyers have filed an application for special leave to appeal the sentence of life imprisonment to the High Court after the NSW Court of Criminal Appeal refused, saying his case was without merit.

Barrister Michael Lee SC, who made headlines representing former political staffer James Ashby in his sexual harassment case against Peter Slipper, intends to argue Ngo was dealt with under the wrong sentencing regime.

Mr Newman was shot dead in the driveway of his Cabramatta home on September 5, 1994 and Ngo was charged with murder in 1998.

After an aborted trial in 1999 and a hung jury in 2000, a third trial in 2001 convicted Ngo of ordering the killing but acquitted the two men accused of the shooting.

The sentencing judge, Justice John Dunford, said Ngo's motive was "naked political ambition and impatience" and his culpability was so extreme that only a life sentence could meet the "community's interest in retribution, punishment, community protection and deterrence".

However, Justice Dunford said Ngo's case was deserving of future review and he would have liked to fix a "very long non-parole period" but he had no power to do so.

In an application filed with the High Court, Mr Lee says Ngo was sentenced under wide legislative changes introduced in 2000, whereas he should have been dealt with under the regime in place in 1994, when the murder occurred.

The criminal justice system recognises that the indeterminate nature of a life sentence "deprives a prisoner of any fixed goal to aim for, it robs him of any incentive and it is personally destructive of his morale".

With that in mind, Mr Lee says it is vital any life sentence be imposed according to law, and determining which of the two sentencing regimes should have been used in Ngo's case is of "general public importance".

Alternatively, Mr Lee questions whether Justice Dunford's "two-stage" approach to sentencing Ngo, in which he looked at the nature of the crime before considering subjective features such as Ngo's age, prior record and good character, is still correct in the light of the landmark High Court judgment in Muldrock in 2011.

In a submission filed with the High Court last month the prosecution said even if the earlier sentencing regime should have been applied, the circumstances of Mr Newman's murder meant it was within the "worst case category of murder" and no sentence other than life could have been imposed.

In June last year the CCA refused Ngo more time to seek permission to appeal, saying the application was made 11 years out of time and the explanations given for the delay, such as his health problems in 2008 and 2009, the death of his brother in 2010 and time spent applying for transfers within the prison system were "entirely unconvincing".

A date is yet to be set for Ngo's special leave hearing.