NSW Premier Gladys Berijiklian is without a parliamentary secretary after the shock resignation of Liberal MLC Catherine Cusack.

Divisions within the government are beginning to show, with the premier's office on Thursday announcing it had accepted Ms Cusack's resignation after an explosive email was leaked to the media.

The email, sent by Ms Cusack to the premier following a factional meeting on Wednesday night, strongly criticised the makeup of Ms Berejiklian's new cabinet.

"If the situation was not already offensive enough ... if you ever say again you made these decisions 'on merit', I swear I will resign from the Liberal Party and join the cross bench," Ms Cusack reportedly wrote.

She also took aim at Energy Minister Don Harwin, whose controversial promotion to cabinet has already ruffled feathers within the party.

"He's easily flustered; he's wafer thin skinned. This is all resulting in gross embarrassment in the upper house," Ms Cusack said in the letter, according to Network Ten.

Ms Cusack took over the role from Scot MacDonald in the January reshuffle that followed Mike Baird's shock decision to quit politics.

She was also parliamentary secretary for education.