Carle is moving to City - subject to a successful work permit application - after failing to impress at Turkey's Genclerbirligi.
"He is versatile and he has quality," said Johnson. "I think we are getting exactly what we need at just the right time.
"But we are also looking to the future and he has already shown he can play at the top level in Turkey and with the Australia national side.
"It makes sense to look ahead. When we were in League One we signed players who we thought could also play in the Championship when we got there."