An inquest into the death of Queensland toddler Mason Jet Lee will not be about "vengeance or blame", when it is held in March next year, a court has been told.

The date was formally set this morning for the coronial inquiry, which could run for up to three weeks and hear from a string of experts, doctors, police officers and Child Safety Department staff.

In the Counsel-Assisting's opening address, before a pre-inquest conference in Brisbane, Dr Jacoba Brasch QC told the court she will ask witnesses to speak "candidly" about what can be done to prevent future tragedies.

"There is nothing to be gained from approaching any witnesses with retribution or revenge — for nothing will bring back Mason, and such emotions cloud the ability to learn," Dr Brasch told the Coroner.

"One must assume each of these witnesses, especially the child safety workers, have deeply searched their soul as to what they might have done differently. But your Honour, this is not a time for vengeance and blame."

Mother and partner known to protective services 'as young people'

The Coroner's Court heard 21-month-old Mason Jet Lee died an horrific death before his "lifeless body" was handed to ambulance officers on June 11, 2016.

"In the middle of winter, without the immediacy of love, care and affection, effectively alone, Mason Jet Lee died what must have been an unimaginably painful death," Dr Brasch told the court.

The court heard a neighbour described Mason as "like a hostage" of William O'Sullivan. ( ABC News )

Mason's mother, Ann-Maree Lee has been convicted over the death, on two charges of manslaughter and child cruelty, and was sentenced to nine years in jail in February this year.

The court heard the case against Lee was one of "serious neglect".

Her partner, William O'Sullivan was also convicted and sentenced to nine years in jail on two counts of manslaughter and child cruelty and was responsible for inflicting the blow to Mason's abdomen around June 6, 2016, which ultimately lead to the toddler's death.

The court heard today, Lee and O'Sullivan were known to child protectives services, both as young people and as adults.

"Lee and O'Sullivan were known to the police, in particular, with their respective long histories of violent relationships," Dr Brasch said.

"From about 14 years of age, Lee experienced a successions of brutally violent partners — facts not too difficult to find.

Anne Maree Lee has been sentenced to nine years in jail for child cruelty and manslaughter. ( ABC News )

"In the year prior to Mason's death, Lee ... [was] homeless, living in a 'friend's' garage, a friend who may have hit her own baby."

Ms Brasch said drugs and alcohol were also features in Lee and O'Sullivan's lives.

"O'Sullivan's florid mental health was something not too hard to find out, including that in April 2015 he had presented to a hospital in the company of police with suicidal and homicidal ideations ..." Dr Brasch said.

"In terms of child protection, both Lee and O'Sullivan raised many red flags for concerns."

William Andrew O'Sullivan had presented at hospital with suicidal and homicidal ideations, the court heard. ( ABC News )

But she said, despite this, little was known about Mason Jet Lee.

The court heard recorded sightings by child protection and support services were "few and far between", and in the "critical week" leading up to his death, he may not have been seen at all, despite support being offered to his mother.

"It does not even seem clear that those services — tasked, indeed obliged, to protect children — even knew Mason was in the primary, if not sole, care of O'Sullivan, largely to the exclusion of his mother," she said.

"This was probably as a means of O'Sullivan exerting controlling and coercive power over her, so she would not leave."

The court heard it seemed "plain" that Lee did not tell support services Mason was in the care of O'Sullivan, but rather a neighbour who said the toddler was "like a hostage" of O'Sullivan.

"It may well become open on the evidence that Mason was hidden from view and Mason was forgotten," she said.

"If that is so, then we all must strive to ensure that children, their voices and experiences are at the forefront of all care and protection and decision making."

Witnesses called to give 'candid' evidence'

Dr Brasch said she would ask the front-line witnesses to "candidly and frankly" speak on what can now be done differently to better protect "our most vulnerable — our children".

"It may be, Your Honour, that all the pieces of the puzzle that was Mason Jet Lee, were in the grasp of authorities, or otherwise easily ascertainable," Dr Bausch.

"And it may well be that no-one put those pieces together to render the picture of a little boy who needed care and protection, without a parent willing and able to do so.

"If so, a fundamental task for this inquest is to identify whether the entities involved in protecting children can be better assisted to render the whole picture of a child in need."

The inquest was set down for March 16, 2020.