Kathleen Folbigg letters and diary entries released to inquiry reveal dark moments in jail

Kathleen Folbigg's letters have been released during an inquiry examining the deaths of her four children.  (AAP: Peter Rae)

An inquiry examining the convictions of Kathleen Folbigg for killing her four children has released letters the mother wrote from jail about the "black moods" referred to in her diaries.

A second judicial inquiry is looking at the convictions of Folbigg, 55, for the deaths of Sarah, Laura, Caleb and Patrick, who were all under the age of two.