Premier Gladys Berejiklian has guaranteed Transport Minister Andrew Constance will remain in the portfolio until the next state election despite him coming under fire for his handling of the rail dispute.

While the boss of the main rail union has kept up his attack on Mr Constance, Ms Berejiklian said he had "done a great job" and the state was lucky to have him as Transport Minister.

And when asked whether he would still be in the portfolio by the time of the next state election early next year, she replied: "Absolutely."

"He has my full and abiding confidence and in fact he has my gratitude because I know how difficult it is to have these discussions," she said of her factional ally.