Queensland’s sentencing laws should be changed to ensure criminals such as the man who killed toddler Mason Jet Lee get what they deserve, say child protection advocates.

Hetty Johnston, founder of child protection organisation Bravehearts, says she is appalled by the nine-year sentence handed to Mason’s stepfather, William Andrew O’Sullivan. Last week at his sentencing hearing before the Queensland supreme court, O’Sullivan admitted to beating the 22-month-old so severely that he died from sepsis caused by his injuries.

The child was left in agony for days as he slowly died, with O’Sullivan “doing nothing” to help the boy, said the chief justice Catherine Holmes.

Johnston is devastated but not surprisedby the manslaughter sentence, which could see O’Sullivan walk free in four years, with time already served.

“I think the justice system is not really the justice system – it’s the legal system, and what we want is some justice for these kids,” she told reporters on Friday.

Johnston said there was little point in the government appealing O’Sullivan’s sentence because judges’ hands were tied.

“I say throw [the system] out and start again, otherwise we’re just tinkering at the edges.”

Johnston says the community should not blame the courts, as the judges are working within a broken framework.

“What [the attorney general] needs to do is rewrite the legislation, put some standard minimum sentences in place,” she said.

The state opposition’s child safety spokeswoman, Ros Bates, said, “I’m sure that the public don’t think that a sentence that lenient is sufficient for what that little boy went through.”

The attorney general, Yvette D’Ath, has three weeks to lodge an appeal against the sentence.

“As with all cases, the director of public prosecutions is looking at the judgement and will advise the attorney general whether there are any prospects for appeal,” a department spokesperson said.

O’Sullivan was also convicted of child cruelty after failing to seek medical care when the boy suffered a broken leg and severe injuries six months before he was killed.

In June 2016 Mason’s final hours were spent wrapped in a towel, lips blue, making grunting noises. After he died, traces of methamphetamine were found in his blood.

O’Sullivan, who had long battled an addiction to drugs, particularly ice, tried to cover up his involvement in Mason’s death by blaming paramedics for responding slowly. The paramedics had taken six minutes to arrive after being called by a friend.

He also told police that he found Mason with his lips blue and mouth clamped on a bottle before calling an ambulance. Later he claimed that his 12-year-old “serial killer” daughter may have beaten Mason.