Prime Minister Malcolm Turnbull says he has taken on "the cause" of getting Western Australia a fairer deal on GST amid new claims the state will lose another $1.9 billion of the national carve-up.

Prime Minister Malcolm Turnbull says he has taken on "the cause" of getting Western Australia a fairer deal on GST amid new claims the state will lose another $1.9 billion of the national carve-up.