Kathleen Folbigg's ex-husband, Craig, has died of a heart attack.
Mr Folbigg, who had also been suffering from an unspecified type of cancer, was rushed to Maitland Hospital on Saturday and died on Monday.
Kathleen Folbigg, 56, was convicted of three counts of murder and one of manslaughter in 2003 after her babies Patrick, Sarah, Laura and Caleb died in suspicious circumstances between 1989 and 1999.
She was granted an unconditional pardon and released from jail in June last year after an inquiry heard there was reasonable doubt about her guilt following new scientific discoveries.
In a final report released in November, inquiry commissioner Tom Bathurst KC found there was an "identifiable cause" for three of the deaths and Mrs Folbigg's relationship with her children did not support the case that she killed them.
Mr Folbigg, who had testified against his wife at her trial in 2003, had said he was "frustrated and disappointed" by the decision to pardon his ex wife.
He refused to provide a DNA sample to scientists examining genetic sequencing, and did not attend or take part in the final inquiry in his ex-wife's convictions.
"He is the victim of this case," Mr Folbigg's lawyer Danny Eid said.
'He is frustrated and disappointed he has to endure yet another chapter.
Mr Eid told News Ltd on Tuesday that Mr Folbigg had been "a dedicated, hard working family man and he spent his last years under enormous stress and pressure".
"He never stopped believing his children had been murdered," he said.
A statement released on behalf of Kathleen Folbigg by her lawyer Rhanee Rego said "there were only two people on this earth who knew what it felt like to lose Caleb, Patrick, Sarah and Laura. My condolences go to the loved ones Craig leaves behind".
Ms Folbigg had been living with friend Tracy Chapman on the north coast following her release from jail.
She announced in February that she had moved back to Newcastle as she adjusted to life beyond her quashed convictions.
"I am forever grateful to my dearest sister friend Tracy for giving me a safe and secure home to come out to when I was released from prison on that fantastic day," she said.
"The time that I spent there reuniting and reconnecting with her and her family are treasured memories.
