“Exciting family news,” Mr Perrottet said in a post on Facebook tonight.
“Our family photo wall is going to need another frame! Helen and I are thrilled to announce we have a little girl due next year.
“With five sisters already, William is looking on the bright side — more bench depth on the family basketball team.”
Mr Perrottet also shared two photos with the post: one of his wife adding an empty seventh frame to a wall beside photos of their other children, and one with their only son sitting in front of the pictures.
Mr Perrottet, 39, became premier earlier this month, taking over from Gladys Berejiklian.
He is the youngest ever to lead the state.