The loved ones of Allison Baden-Clay have welcomed the decision to reinstate her husband Gerard's murder conviction over her 2012 death.

The five-judge bench of the High Court this morning unanimously ruled in favour of an appeal by Queensland prosecutors to overturn a decision to downgrade Baden-Clay's conviction to manslaughter.

Outside court, Allison's close friend Kerry-Anne Walker said the nation's highest court had confirmed what those closest to her knew from the moment she disappeared.

"Gerard Baden-Clay murdered his amazing wife Allison," she said, a yellow ribbon pinned to her clothes.

"Today's decision brings to an end Gerard's attempts to smear Allison's name."

Gerard and Allison Baden-Clay: Murder case in review View Gallery

Ms Walker said her late friend's legacy would endure through her three daughters and described her as a loving wife and mother.

The decision also marks the end of a legal process which has captured public and media attention since Baden-Clay was charged in June 2012, just weeks after his wife's body was found on a creek bank some 13km from her home.

The Court of Appeal's controversial decision to replace the former real estate agent's conviction with manslaughter in December last year drew scores of protesters to King George Square in Brisbane's CBD.

But Queensland Law Society president Bill Potts said today's outcome showed Australia's legal system was working.

Allison and Gerard Baden-Clay.

Allison Baden-Clay.

"The wheels of justice grind slowly but they do grind exceedingly fine," he said.

The High Court found the lower court made a mistake when it concluded the jury's verdict of guilty of murder was unreasonable on the basis the prosecution had failed to exclude the hypothesis Baden-Clay had unintentionally killed his wife.

"The hypothesis on which the Court of Appeal acted was not available on the evidence," the judgement read.

"(The court's) conclusion ... was mere speculation or conjecture rather than an acknowledgment of a hypothesis available on the evidence."

Mourners at a memorial for Allison Baden-Clay. (AAP)

Scratches on Gerard Baden-Clay's face were discussed in court. (AAP)

For Baden-Clay's former lawyer, Peter Shields, the day marked the end of an ongoing legal and personal saga for both Gerard and Allison's families.

He said the High Court's decision would have "massive" implications for how criminal trials were handled in the future and conceded the ruling would have been different if his client had not given evidence of his own.

"He has accepted the decision, as he must," Mr Shields said, indicating he planned to meet with Baden-Clay on Friday afternoon.

He is currently serving a sentence of life imprisonment with a non-parole period of 15 years.