The account of his exploration is thus briefly given by Flinders (Terra Australis volume 1 Introduction page 61) After relating the arrival of his two ships off Cape Inscription at the north end of Dirk Hartog's Island he proceeds: No mention is made by Valentyn of the ships entering the road, nor of their departure from it; but it should seem that they anchored on February 4th.

On the 5th Commodore Vlaming and the commander of the Nyptang went with three boats to the shore, which proved to be an island.
They found also a river, and went up it four or five leagues, amongst rocks and shoals, when they saw much water inland, as if the country were drowned, but no men, nor anything for food, and wherever they dug the ground was salt.

They afterwards came to another river, which they ascended about a league, and found it to terminate in a round basin, and to be entirely salt water.

No men were seen, nor any animals, except divers, which were very shy; and the country was destitute of grass and trees.

Returning downward on the 10th, they saw footsteps of men and children of the common size, and observed the point of entrance into the river to be a very red sand.) March 19.
The wind still blew pretty fresh from the southward; we however had no surf to impede us and therefore got under weigh soon after dawn.