Liberal MLC Catherine Cusack has resigned as Parliamentary Secretary after sending Premier Gladys Berejiklian an email strongly criticising the makeup of the new cabinet.

Ms Cusack's email singled out Energy Minister Don Harwin, who entered cabinet in the most recent reshuffle and was also made leader of the government in the Legislative Council.

Catherine Cusack has resigned as parliamentary secretary

"We are working with Don's dreadful performance. As you know, he is no orator," the email said, according to Channel Ten.

"He's easily flustered; he's wafer thin skinned. This is all resulting in gross embarrassment in the upper house."