The Gemara in Shekalim 3a seeks to prove the concept of Hefker Beith Din Hefker. To that end, it cites the incident in which Ezra decreed that anyone who refused to gather in Jerusalem would forfeit his assets. But this seems to me to just show that he employed a coercive tactic, not that he was right to do so, nor that this tactic was appropriate or legitimate. How does Ezra's decree prove Hefker Beith Din Hefker?
Ezra was comparable to Moshe, and would have given the Torah had Moshe not preceded him, so it is not unreasonable to say that his actions were legitimate.
Anyway, it is also seen from a posuk in Yehoshua, in Yevomos 89b.