Kathleen Folbigg has broken down while giving evidence about her diaries at an inquiry into her convictions for killing her four children.

Key points: This is the first time Folbigg has spoken in court about the diaries

This is the first time Folbigg has spoken in court about the diaries She smiled at supporters when she was brought into the NSW Coroner's Court

She smiled at supporters when she was brought into the NSW Coroner's Court Folbigg denied disposing of some of her diaries because entries about her dead children were "incriminating"

Folbigg is serving a minimum 25-year sentence for murdering three of her children — Sarah, Patrick and Laura — as well as the manslaughter of her first-born, Caleb.

She told the inquiry in Sydney that she never lied to police about the diaries she kept from 1989 to 1999 which were used to convict her of killing her babies.

Folbigg, who broke down in the witness box, also denied killing her children.

"I don't know why any of my children died, but I didn't kill them," she told the court.

"I didn't kill my children. And these diaries are a record of just how depressed... and [what] a struggle I was having."

It is the first time Folbigg has spoken in court about the diaries as she chose not to give evidence at her 2003 trial.

She wiped away tears when asked about a diary entry in which she wrote that she was worried about being alone with her child.

The 51-year-old explained that her diary entry was referring to being "scared to death of not finding my child alive".

While questioned by the Director of Public Prosecutions barrister Christopher Maxwell QC, she denied disposing of some of her diaries because entries about her dead children were "incriminating".

"I have never hid my diaries," she told the court.

"They were always in places where people could see them."

The inquiry heard that Folbigg wrote in the diaries about her children "pushing her button" and causing her to "snap her cog".

She denied Mr Maxwell's suggestion that "snap her cog" meant she intentionally harmed her daughter Sarah and "tried to stop her living".

"I don't agree with that at all," Folbigg said.

She rejected another suggestion by Mr Maxwell that a diary entry four years after Sarah died was "heartless".

The entry read "I miss her ... but I am not sad that Laura is here and she isn't".

She explained it was a diary entry comparing her children and reflecting on them.

"I was constantly doubting my ability as a mother," she said.

Kathleen Folbigg told the inquiry she "was constantly doubting her ability as a mother". ( Supplied )

She also told the inquiry some of her entries were about her frustration with being a parent and were written because she needed to "vent".

The inquiry heard about one entry that stated the dead children would no longer have to "make decisions" or "deal with stress".

Folbigg's evidence to the inquiry is restricted to her diary entries and the disposal of those diaries.

The inquiry has previously been told DNA material taken from Folbigg's children has found nothing in their genes that could explain their sudden death from natural causes.

Folbigg's former husband Craig was also in court — he is being represented by high-profile barrister and former crown prosecutor Margaret Cuneen SC.