Gerard Baden-Clay is a murderer.

The High Court of Australia has reinstated his conviction for the murder of his beautiful wife, Allison, and in doing so set an important precedent for future cases, according to lawyers.

For Ms Baden-Clay's friends, the decision came with "both relief and elation".

After more than four years of limbo the former real estate agent had run out of options for appeal and would serve life in prison, with the parole board responsible for his release date.

Baden-Clay told his original murder trial he had nothing to do with his wife's death, in April 2012, and denied fighting with her, killing her and disposing of her body in Kholo Creek, 13 kilometres from their Brookfield home in Brisbane's west.

On appeal, his lawyers argued for the first time that a scenario where he intentionally killed her was a possibility that hadn't been addressed by the prosecutors.

The Court of Appeal downgraded the murder conviction to manslaughter, prompting a public outcry, which culminated in a hundreds-strong rally in King George Square, before the High Court agreed to hear a rare appeal of that decision.

On Wednesday, the court handed down a unanimous decision just one month after hearing arguments, throwing out the Court of Appeal's ruling and leaving Baden-Clay with his initial sentence of life behind bars.

For Kerry-Anne Walker, who spoke on behalf of Ms Baden-Clay's friend and family, it was common sense prevailing and proof that "the ignorance lay elsewhere", not with Queenslanders who objected to the downgrade.

"The law has acknowledged what we, who were closest to her, knew from that very morning Allison went missing - that is - that she was murdered," she said, outside the High Court in Canberra.

"Gerard Baden-Clay murdered his amazing wife Allison.

"The evidence in the original trial displayed his intent as well as his character.

"Today's decision brings an end to Gerard's attempts to smear Allison's name. If some were in doubt as to his true nature, his behaviour after Allison disappeared and during the trial must have removed that doubt."

It was a "red letter day", according to Queensland Law Society president Bill Potts.

"What this simply means, is the justice system works," he told Nine News.

He called on politicians and the public to respect the High Court's decision in "arguably one of the most high-profile and emotive cases in Queensland's legal history".

"The original decision was in-line with the understanding of the law at that time, and the High Court's decision reflects the current interpretation of the law," he said, in a statement.

He said it was now up to the Queensland Parole Board to decide if Baden-Clay should ever be released.

"This is a decision not just for Queensland or for the Baden-Clay family, it is a decision for all Australians.

"The way in which circumstantial evidence may be used and post-offence behaviour may be used has now been clarified by the High Court.

"That, in turn, can be utilised across a range of cases that are waiting in the wings.

"This is an important precedent."

Baden-Clay's lawyer Peter Shields was disappointed in the outcome and said the High Court's decision would likely have been different if his client hadn't given evidence at his own trial.

He said the decision would have "massive" consequences for every trial in the country, particularly in relation to whether defendants chose to give evidence.

"It must affect every criminal trial throughout the Commonwealth," he said.

"The High Court decision that was handed down today is now in place and will affect trials that commence tomorrow."

The High Court found the Court of Appeal erred when it found a jury's verdict of guilty of murder was unreasonable because the prosecution hadn't excluded the hypothesis Baden-Clay unintentionally killed his wife.

It found the jury was entitled to consider the whole of the evidence to satisfy themselves he'd intended to kill or inflict grievous bodily harm on Allison.

Allison's body was found on a creek bank in April 2012, 10 days after her husband reported her missing from their Brisbane home.

Baden-Clay was charged with her murder about two months later.

- With AAP and Nathanael Cooper

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