And Joshua said to all the people, "Thus says YHVH, the God of Israel, 'Long ago, your fathers lived beyond the Euphrates, Terah, the father of Abraham and of Nahor; and they served other gods....'"
Joshua 24:2

In this passage, Joshua shows himself to be a prophet, delivering a direct word from YHVH. "Thus says YHVH, Elohim of Israel…"

A prophet has no inherent authority in his office except when he is acting as a conduit for the words of God. His prophecies must be heeded, but only because they are the word of YHVH. However, a prophet might also be a teacher, a priest, a general, a judge, or, like Joshua, a combination of two or more of these offices, and those do cary varying degrees of authority.