Roy Morgan poll some 3200 voters each time and if you understood statistics you would know that provides a very good confidence level that is proven scientific valid.
Now add the sample over 19 months the have done some 20 polls x 3200 each = 6400. Not such a small sample.
Now add up the sample of the other poll who have consistently produced similar results to Morgan.
Now also remember that newspoll and Morgan have consistently predicted the outcome of election with in a few points of a %
