ACT Attorney-General Gordon Ramsay last week introduced a bill to make changes to key child sex offences, to help secure a surer path to successful prosecutions.

The changes create a new offence for engaging in a "sexual relationship" with a child, based on prosecutors specifying an overall period of the "relationship" and two cases of abuse within that period.

Mr Ramsay told the Assembly the changes would allow prosecutions consistent "with the ways in which victims remember abuse" and that its retrospective effect was "in recognition of the fact that delay is a typical, rather than a peculiar, feature of child sexual abuse".

But the ACT Bar Association is concerned the new offence could negate the need for all individual charges to be "particularised" through specific details of individual charges, a fundamental legal requirement to give defendants a fair trial.

Bar president Ken Archer said in any normal criminal trial, the prosecution had to prove beyond reasonable doubt that a defendant had committed the specific offence charged in the period claimed.

But he said the new offence "enabled the prosecution to prove the offence notwithstanding they don't have to particularise the actual offence that's been sid to have been committed".

Mr Archer also said its retrospective operation could allow for a person accused of the offence to be convicted on a charge that did not exist at the time of the alleged crime.

He said the bar had written to the government about their concerns, but since the legislation has been introduced there was now little that could be done to reverse the decision.

Defence lawyer Paul Edmonds, who sits on the ACT Law Society's criminal law committee, said he was also concerned about the potential ramifications of the change on anyone falsely accused of such crimes.

"Not every allegation of child sexual abuse will be true, just as we've seen some quite notorious recent cases of false allegations of domestic violence, where an alleged victims was convicted of making a false allegation," he said.

"What it comes down to is that if a jury is told an accused has done something ten times but they only have details of one or two charges, can anyone seriously say that jury isn't more likely to find the person guilty than if they were only told about the single allegation.

"The right to a fair trial should not depend on the type of offence being prosecuted, but this [proposal] potentially means that there will be a less fair trial if you're accused of a child sex offence."

The government consulted the bar, Legal Aid ACT, the Canberra Rape Crisis Centre, the ACT Human Rights Commission and the Chief Justice of the Supreme Court and Chief Magistrate on the changes.

While a government spokeswoman said the government made some key changes to the legislation based on those talks, she said the issues the bar raised were "fundamental policy objections" to the Royal Commission's recommendations and were not "technical issues" or unique to the ACT.

"The Royal Commission's recommendations serve the interests of the community in holding perpetrators of sexual abuse responsible, and in treating survivors of abuse fairly in the court process," she said.

She said the new offence "will absolutely not mean people falsely accused may be more easily prosecuted".

"What these amendments mean is that in cases where abuse was so prevalent that a survivor's memory cannot identify a range of specific incidents, the offender cannot use that fact to avoid conviction," she said.

"The community and survivors of abuse have a right to a fair hearing that is enhanced by this amendment."