Family of Allison Baden-Clay await High Court decision on downgrading of husband's murder conviction

The loved ones of Allison Baden-Clay today wore yellow to the High Court, which has been asked to decide whether to reinstate her husband Gerard’s murder conviction for killing her.

It is expected to be months before the five-judge bench reveal their ruling, after hearing arguments for and against the controversial downgrading of the former real estate agent's murder conviction in Brisbane today.

The Queensland director of public prosecutions successfully launched a bid to have the case considered by the nation's highest court after a shock decision by the state's Court of Appeal to substitute it for manslaughter last December.

Roughly two years after a jury found Baden-Clay guilty of the murder of his wife, her sister Vanessa Fowler said her family had been overwhelmed with support.

"It continues to be a very long and challenging road," Ms Fowler said outside court, wearing a yellow ribbon in honour of her sister.

"However (we) are determined to achieve justice for Allison."

During the hearing, former solicitor-general Walter Sofronoff argued the Court of Appeal made an error by finding there was no evidence Baden-Clay had a motive to kill his wife.

“He had pronounced and affirmed his love for (his mistress) verbally and in writing as late as days before he killed his wife,” Mr Sofronoff said.

“(He) had found living with his wife intolerable and unendurable.”

Mr Sofronoff claimed the murder was “not only calculated but cold blooded” and that Baden-Clay “is a man who is capable of murder”.

He also challenged the court's conclusion that Baden-Clay's behaviour after his wife's death was neutral on the issue of intent and that there was a reasonable hypothesis of manslaughter.

But defence barrister Michael Byrne said the case was not one where there had been pre-existing violence or threats and claimed Baden-Clay's financial woes and extra-marital affair were merely pressures, not motive.

However, he was subjected to rigorous questioning from the bench about how the scenario of an accidental death remained open despite Baden-Clay's own evidence at his trial directly contradicting it.

In particular, Justice Patrick Keane remarked the testimony was "inconsistent with any notion" of an unintentional killing.

Outside court, Queensland Law Society president Bill Potts welcomed the High Court's consideration of the high-profile case.

"Quite frankly, (it's) one of the great benefits of living in a free, liberal democracy," he said.

A decision is expected in three to six months.

The court can decide to reinstate the murder conviction, maintain the manslaughter one or, in a very unlikely outcome, order a retrial.

Mrs Baden-Clay body was found on a creek bank in April 2012, 10 days after her husband reported her missing from their Brisbane home.