In this case it was[1] a huge artificial forest of eucalyptus. They grow pretty fast without those nasty koalas nearby. You can cut them and they regrow from the roots, but after a few times they planted new trees.

I'm not sure if they replaced the local forest with the eucalyptus, but my guess is that the areas was already used to cultivate corn, tobacco, sugar cane or something.

[1] I'm not sure about the current state. My guess is that it is still there, but that steel mill had a complicated story in the last 30 years.