Santa Cruz de Oñez was a fort founded by Martín García Oñez de Loyola in May of 1594, near the confluence of the Bio-Bio and Laja Rivers on the right bank of the upper reach of the Rele River in Catiray, ten kilometers south of the Bio Bio. The site was in an elevated location but with a shortage of water. Gold mines were located across the Rele on a stream called Millapoa.
The fort was elevated to the rank of city in 1595 giving it the name of Santa Cruz de Coya, or Millacoya.