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.
"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."
Mr Harwin declined to comment.
Mr Harwin was instrumental in marshalling the numbers for Ms Berejiklian to become Premier after the resignation of Mike Baird.
Ms Cusack attacked Ms Berejiklian's statement that the cabinet was appointed solely on merit and threatened to quit the Liberal Party and move to the crossbench if she repeated the claim.
"If you ever say again that you made these decisions on merit I swear I will resign from the Liberal Party and join the crossbench," she wrote.
In a statement on Thursday afternoon, Ms Berejiklian said: "I have this afternoon accepted the offer of resignation from Catherine Cusack MLC as Parliamentary Secretary."
Ms Cusack declined to discuss the details of her email, but told Fairfax Media: "All I want to do is apologise to all the people I have let down."
Ms Cusack was Parliamentary Secretary for Education and the Hunter.
She was the Liberal Party's environment spokeswoman in opposition but was overlooked for a cabinet post when the Coalition came to power at the 2011 election.
