It’s common knowledge now that most of the special effects in Avatar were done by Weta, although new information says that Weta found themselves a bit over their head and had to turn to other studios, namely ILM, to help them keep the deadline.  An article on CBS4 discusses how the two studios worked together both competitively and cooperatively to bring the masterpiece to the screen.

Still, with both houses working in parallel, there was certainly a bit of a race to finish a shot, Knoll said, because the team that was fastest would be able to more or less set the tone for the whole scene. “Whoever gets there first is who drives it,” he said.

“For example, in the home tree sequence, we have to fire a bunch of missiles,” Knoll recalled. “[There wasn’t] anything established for what the missile trails look like. We did our own version of the what [they] would look like and Jim liked it, so that's what Weta had to match.”

Of course, in other cases, Weta would finish first, and ILM would have to match what the New Zealanders came up with. And in some cases, it was a bit of “splitting the difference,” Knoll said. Ultimately, he added, he hopes that audience members won’t be able to tell that two separate visual effects teams shared the work.

via How ILM Rescued Avatar’s Special Effects – cbs4.com.