The High Court has agreed to hear Queensland prosecutors appeal against the downgrading of Gerard Baden-Clay's murder conviction for killing his wife Allison.

The court decided today, in Canberra, to hear the appeal, at a date yet to be fixed.

The Director of Public Prosecutions argue Queensland's Court of Appeal got it wrong when they downgraded the conviction to manslaughter last December, saying there was a strong motive for murder.

The former real estate agent had yearned to be with his mistress, and he stood to gain a life insurance payout, the director said.

Allison's badly decomposed body was found in Kholo Creek in May 2012, over a week after she disappeared from the family home.

Baden-Clay was jailed for life in 2014 but that was quashed in December.

Allison's cousin Jodie Dann watched the High Court decision from the public gallery, but would not comment afterwards.

Attorney-General Yvette D'Ath broke news to sitting MPs shortly after the decision was announced, but also would not comment further as the matter was before the courts.

Allison's friend Nicole Morrison said anyone who was touched by her would be grateful with today's High Court decision.

"So many important questions will now be considered and we steadfastly await the determinations of the court," Ms Morrison said.

"While those proceedings take place, Allison's legacy will continue to be heard through her family and friends and we continue to thank everyone for their ongoing support."

'Let justice do its course'

Queensland Law Society president Bill Potts said Queenslanders could be confident the High Court made the right decision.

"This decision represents the best interpretation of the law, unaffected by bias or the relentless press of the 24/7 media cycle," he said.

"It is important now to allow the final chapter of the legal process to play out according to the practices and procedures of the courts, and without hyperbolic media commentary.

"We need to remember that whatever people think of the result, this is the final act of a tragedy in which three daughters have lost a mother, two parents have lost a daughter, siblings a sister and we should not, as a community, allow this to turn into a sideshow."

There was huge support for the Director of Prosecutions to appeal, more than 73,000 people signed a petition and thousands rallied in Brisbane's King George Square when Baden-Clay's conviction was downgraded in December.

His lawyers argued there was not enough evidence to prove he intentionally killed Allison, who may have fallen and hit her head during an argument.

They then suggested a hypothesis that he covered up the death, by dumping it in a local creek, out of panic.