Former High Court judge Michael Kirby says it could be unconstitutional to tell juries about defendants' previous crimes.
Queensland's Attorney-General Jarrod Bleijie has said he'll look at a range of possible reforms suggested by the state's Chief Justice Paul de Jersey.
Justice de Jersey has said juries should be allowed to hear about defendants' "relevant criminal histories".
But Justice Kirby says that could be unconstitutional and would make it harder for defendants to get a fair trial.
"Well that is quite contrary to the common law tradition," he told ABC radio.
"The common law tradition is that we don't try a person for being a bad person, we try a person for an offence.
"And we require the state, the Crown, to prove that offence against the person."
Civil libertarians have also warned any such change will result in the miscarriage of justice.
Mr Bleijie has said there's no formal application for parliament to consider Justice de Jersey's suggestions, which also include compelling defence lawyers to reveal their tactics before a trial starts and allowing judges to explain to juries the legal meaning of "reasonable doubt".
But the attorney-general said he would look into the issues if needed.
"There is no doubt that these contemporary, and some may say controversial reforms, would require a large mind-shift of those practising in criminal law in Queensland," Mr Bleijie said earlier this month.
He said the proposals would allow for "greater transparency in criminal trials".