Child sex offenders could face life behind bars under laws to be re-introduced to federal parliament next week.

Predators who commit serious crimes against children could also face mandatory minimum sentences, while repeat offenders would find it much harder to get bail.

The vast majority of child sex offences are state crimes, so the proposed shake-up of Commonwealth laws is largely aimed at paedophiles who offend online or overseas.

Attorney-General Christian Porter took aim at penalties handed to paedophiles.

“It simply beggars belief that nearly a third of all child sex offenders who were sentenced last year were not required to spend a single day behind bars,” Mr Porter said on Tuesday.

“When jail terms were handed out, the average length of time that offenders spent in custody was just 18 months.”

The Coalition’s announcement came as it emerged that one of Australia’s most notorious child abusers, Michael Guider, will be released from jail within days.

The now 68-year-old pleaded guilty in 2002 to the manslaughter of nine-year-old Samantha Knight, who disappeared from Bondi on August 19, 1986. Her body has never been found.

After his 17-year jail term expired, Guider was placed on an interim detention order that expires on Thursday.

On Tuesday, NSW Supreme Court Justice Richard Button rejected an application to keep Guider behind bars for another year, instead imposing a five-year “stringent” extended supervision order with 56 conditions.

Outside court, Samantha’s mother, Tess Knight, said she was angry at the decision.

“I see the effect that this has had on the people he abused as children, who are now young adults,” Ms Knight said.

“They have to live with the thought of him walking around having the freedom they don’t have.”

Lisa Giles, who was abused by Guider as a child, said: “A child molester and a child killer will be released among us this week.”

The Coalition tried in 2017 to pass similar legislation toughening sentences for those who offend against children. But it was stymied after Labor baulked at the inflexible nature of the mandatory sanctions.

The federal opposition argued juries would be less likely to convict if they knew judges had no discretion on sentencing.

However, Labor now appears open to reconsidering the government’s bill.

“Every child must be kept safe,” shadow attorney-general Mark Dreyfus said.

“Labor strongly supports keeping children safe and holding these horrendous individuals to account.”

The legislation will be introduced to parliament next Wednesday.

-with AAP